i have had a problem with my knees since i was very small, i'm now 43. occasionally when i turn over in bed it feels like the bone or cartlidge in my knee shifts out of place. i have to straighten my leg out and i hear a loud pop like the bone goes back into place. it is very painful. this happened again early this morning but it wouldn't go back in. i layed in bed with my pillow under my knee because i can't straighten it. it now feels like my tendons are very tight and i still can't straighten my leg because of the pain. any ideas of what might be going on?