I have a very painful lump on the inside (medial side) of my left knee. It's as if there is a marble under my skin and i can move it around. It doesn't hurt as much when i'm standing/ walking around, but after i sit/relax for about 10 minutes it gets EXTREMELY sore (stabbing/throbbing pain in the whole joint). I wake up to my knee throbbing everynight/morning. I have a history of ACL tear (fixed) in same knee w/ meniscus tear on medial side: March 2004.(I see an Orthopedic MD June 22nd)What could it be?