tom!! This happens to me all the time because the ligaments that hold my knee-cap in place are very loose. WHen it happens to me usually im in serious pain and then it goes after 3 - 4 hours. Those thigns that hold everythign in place round ur knee work wonders.
However ,i did this at download and it went swollen, and it still is :lol: . I went to the hospital about it yesterday and it turns out when i twisted my knee a bit of bone got chipped off the back of my knee-cap. Need surgery on it now :roll: .
Not to scare you, just my experience. If the swelling doesnt go down then id get it seen to, but the pain should go in a few days. And just aheads up, dont go to a minor injuries clinic or ur GP, because i did both and there really is nothing they can do :lol:
Best of luck mate!
Chances are you will be fine
Petre