@bcool123
I get this and my teen son does too. Sitting on a hard surface, like a toilet seat, can compress blood vessels affecting circulation and also pinch/irritate nerves from hips/buttocks through thighs/hamstrings. The numbness resolves once you stop the compression and improve circulation.

Using a heating pad may increase inflammation and put pressure on blood vessels/nerves so that may add to the numbness. Something that would be cooling may be better.

Sitting too much is not good and can pinch nerves/blood vessels in knee joints, too.