Foot Pain!

I need help getting at least an idea of whats wrong with my feet. I've been to the doctor for a knee injury and a shoulder injury in the past month and I hate to go so often. I'm getting sudden, sharp pain in the back of my foot/ankle. It is mostly in my right foot. It does not always hurt but once it starts hurting its hard to walk, and hurts a lot to go down stairs. It generally gets better when I rest, although if I have my feet up on a hard surface the same area seems to get stiff and hurts a bit when I try to get up.