Does your guitar cause you pain in your right leg?
If you get pain in your right leg, it might be your guitar that's causing the pain in the first place. Fortunately, there's an easy way to fix it.
Most guitar players are right-handed and play right-handed. Personally, I'm left-handed but play right-handed because that's just how things happened to work out.
It's probably true that when you play sitting, you don't use a guitar strap, and most if not all of the guitar's weight rests on your right leg.
You need to get the guitar weight off that leg. Fortunately, there are a few easy ways to do this.
Solution 1: Use a lightweight guitar
The easiest way to go lightweight with a guitar is either to use a semi-hollow body or an acoustic.
If I were personally going to switch over to semi-hollow, I'd get the Epiphone ES-339, which is a semi-hollow in a Les Paul-sized body.
Solution 2: Play standing with strap
Solution 3: Play sitting using a strap
One of the best straps for comfort playing is a soft leather strap. Specifically, the type with one skinny end (which goes over your shoulder) and the other side thicker (which attaches to the rear of the guitar.)
When you play sitting with the strap on, the guitar either has to be lightly touching your leg or elevated so it's not touching your leg at all.
Yes, you may encounter some shoulder soreness at first when playing this way, but you should get used to it. If you don't and the shoulder soreness doesn't go away, you will need to switch over to a guitar that is lighter in weight. Either get the parlor acoustic or a semi-hollow.
Does a guitar cause restless legs syndrome?
I'm not a doctor, but the answer is no.
RLS is neurological disorder that typically affects the legs. Common symptoms are best described as feeling like you have an itch you can't scratch, "crawling" feelings and so on. These feelings occur usually when in a relaxed state, such as lying in bed waiting for sleep or just sitting and reading a book.
Where the guitar is concerned, if it's the case where your right leg felt fine before you started playing guitar, but then exhibited numbness afterward, chances are pretty good that the weight of the guitar resting on your leg was the cause of the numbness in the first place.
However, I'll say again that I'm not a doctor, so if you're getting pain in the right leg after playing guitar sitting, you can try the solutions above. If the solutions don't work, see a doctor.