My guess is that you are making sure the guitar is not slipping down your leg. I do this as well unless I have a foot rest - quite similar to the one linked by @theoldman66.
This actually a bad posture for your hip angle and will cause lower back stress. You will eventually cause health issues with your blood vessels in the that leg as well. Get out of that habit. 
Hmmm… That could possibly be the culprit. 
I should try all of those suggestions during the next session. And yeah I’ve been trying to make it a habit to tap my toes to be on beats.
Thanks Thomas for the feedback! 
I see, that could be it. Alright, I’ve never thought that such thing could exist! 
But I guess it exists for a reason. I’ll be sure to look into it. Thanks Ian!
Hmmm I never thought of it like that. That’s probably the case. Yeah I’m planning to play the guitar for a long time to come, so definitely don’t want to cause any back problem in the future.
Alright will do. 
Oh ok interesting, I thought that your body would eventually find the best way for it to play the guitar and you stick with that throughout your life.
And yeah, I’m currently saving up to buy a new amp, so that’s probably a good idea to just use what’s already available.
Thank you so much Joshua for all the feedbacks!
This is a really good thought. I have very short legs, like a 26” inseam. I had to search for and find a special stool that would go low enough for my thigh to be parallel to the floor. I need about a 17” chair height.
Yeah that’s probably the case since that’s what everyone else here implying. Glad to know I’m not the only one doing so.
I see, yeah I never really think of it like that. I surely don’t want to have any of that problem in the future!
Thanks for all the insightful feedbacks Michael!
Yeah it is a good thought. Well I never measured my leg but I guess that could be the reason.
I’ll try to do as you mentioned the next time I play and see if that’ll fixed the problem.
Thanks again!
Do you use a strap? I find that even when sitting, if you have a strap taking the weight of the guitar so that it’s just touching your thigh then you can pretty much do what you like with your legs (taking into account the long term health risks that have already been mentioned above of course).
A decent adjustable height and backrest armless chair with built in footrest will also help with your posture and make you sit up straight and avoid long term back pain issues.
I do not use a strap. I mean I have one, but I thought you only use strap if you were to play standing up, which currently is way beyond my grade.
I’ve never thought of using it while sitting down. I shall give it a try later.
The only chair that I’m thinking about while reading that is the gamer chair or the office chair.
I’ll keep a lookout on affordable adjustable armless chair with a footrest.
Thanks Gordon!
Dean, I’m constantly working on my posture as well (I’m about 1.5 yrs into this journey). I’m relatively short at 5’3” and find that my seating arrangement also changes depending on which guitar I’m playing. For me, both the depth of the body (especially acoustic vs electric) and the size of the bout make a difference. Some days I just can’t get comfortable, so I stand!
I see. I’ve never really thought about posture before. Right now in my journey is all about mastering the basic chords, chords changes and strumming. I’m a beginner so I’m more focused on those things.
I only have ever tried to play on one guitar which is the current guitar that I have right now, so I didn’t think about how the posture will change along with the type of guitar. So thanks for bringing that to my attention.
Haha I can relate! But usually I’ll just take a break and get back to it later. Because I’ve tried playing the guitar standing and it was a bit heavy for me to handle.
Congratulations on your 1.5 years Judi! Hopefully I’ll keep trying and reach that milestone someday.
Thanks again for the feedback!
I don’t think that you have an option to give up.
Hi Judi
I too just use a strap sitting or standing.
You dont have to think about balancing or holding the guitar.
I have mine ( as sairfingers says ) about a 1in above my thigh so I can tap or whatever without worrying where the guitar is going to end up.
The chair I use when playing has legs short enough so I can put my feet flat on the floor, and no arm rests.
I tried a footrest but found it didnt work ( for me ).
Some good other suggestions here too. I’m sure you’ll find one that works for you.
P.S. have a word with the errant leg and tell it to behave.
Mike
Hey Dean,
I suggest too using a strap at all times, sitting or standing. Most do. It’ll secure the guitar at a consistent, regular, and comfortable height.
Cheers, Shane
As suggested by pretty much everyone else, the problem with your right leg could be caused by the height of the chair causing your upper leg to slope downwards. However, I sometimes do the same thing, and it has nothing to do with the height of the chair I’m using (I tried different seating heights, from too high to too low) - it’s simply tension, concentrating hard on my hands. When I do an exercise or play something I’m comfortable with, my leg stays where it should be.
I’m yet another who plays sitting down with a strap…many of us do.
You might also be interested in this topic, which goes into lots of detail on sitting positions, chairs, and so on.
I noticed recently my right foot was on its toes as well, so I got a taller foot stool the other day and it seems to be helping, at least my heel isn’t up in the air anymore. I’m hoping this may also help with the upper arm pain I’ve been experiencing. I guess I’ll have to wait and see. I’ve been struggling with posture for months now.
I can’t imagine your other leg going numb is a good thing. I’d suggest keeping it on a footstool with your other foot.
Hey Dean
I’m also very bad for sitting on my left leg, or both of them cross-legged on the chair and then not being able to walk when I get up 
or crossing one leg and kinda leaning to one side.
But it really does cause a lot of uneven hip tension and lower back strain, and also for me, a lot of tension in my right shoulder. I have found that one of those footstools really helps, but I used a stack of books on the floor before I got it (just as good).
And yes! use a strap even sitting down, also don’t want to accidentally drop your guitar! 
Just an FYI heads up - I recently bought D’addario auto lock straps for my acoustics but didn’t fit the pins on either of them. They’re really good otherwise, you know… if they fit 
Glad to know that I’m not the only one thay does this. Thanks for the suggestion.
Oh no, hopefully your arm will get better!
Alright will do. Thanks Rebecca. 
Hey Jasmine. 
Glad I’m not the only one. Hehe…
I never thought about all of that. So that’s why sometimes my right shoulder ache.
Yeah, I really need to dig out those thicks book because others have mentioned using that as well. 
I see, because I’ve tried using the straps when I first got my guitar, but somehow just makes me uncomfortable. But I guess I’ll have to try it again. And yes, guitar are heavier than they look.
Oh nice. I should look into that. I heard that the brand makes good strings. I’ll definitely check it out. Thanks for all the feedbacks Jasmine!