I get pain on the back of my thighs when sitting. they are not swollen or red, but the pain appears as soon as i sit. moves b/n the knee and buttock.

Piriformis syndrome: You might have a syndrome where one of your nerves gets trapped under one of the muscles as it flattens when you sit. It should get better with exercise that strengthens those muscles and can be temporarily relieved with a piriformis injection. That would be the most benign thing it could be but please get it checked out to make sure.