Don't worry you'll be fine
IME you can feel if something odd happens anyway. Towards the end of pregnancy ds1 headbutted me really hard in the stomach. He actually did me a favour as ds2 flipped over from being breech (not that it mattered really as he ended up be elective section). But boy did I feel him move and he set of contractions. Got checked over and all was fine.
So I'm sure its fine- I fell over quite a lot when pregnant with ds2 (actually more to the point I got pulled over a lot by ds1), but never had any problems. Of course if you're worried get checked out- they won't mind.
Hope your knees are OK.