What Causes Pain In The Back Of Thighs After Prolonged Sitting?

Ive been reading that some people have similar symptoms as ive been experiencing but only a couple sound exactly the same. I am experiencing a burning pain in the back of both my thighs whenever I sit more than 30 or 40 minutes. I have to constantly shift positions, often sitting on my legs because of the burning pain. I do walk and stand throughout most of the day, often because of this burning pain. Its like the burn gets to where I cannot sit any longer so I have to get up and do something. The burning sensation stays on the back of my legs for a while. When I go back to sit after about half an hour, the burning is still there and begins to hurt again. I read that it might be due to lack of vitamin D but that seems kind of strange because I live in Arizona and I get lots of sun light although I don't go out much and recently retired from 31 years of office work. The sitting at work used to kill me and I feel much better because I can get up now and walk whenever I walk but it seems like its getting worse. Any information or advice will be appreciated. Thank you so much.