If it is just a superficial bruise over your knee cap, you should be fine. But if it hurts to sit down (take weight off your knee) and flex and extend it, I'd ice it and stay away from vigorous exercise for a couple of days. This could mean you actually bruised the bone or perhaps even fractured it (of course, you'd be in serious pain and probably would have gone to the doctor had you fractured it). However, if it does not hurt to bike, nor is the joint stiff (from swelling), go ahead with it.
The tender sensation on the back of your knee is a little worrisome, but if you are more concerned about the knee cap, I am sure you will be fine. It just all depends on how exactly you fell. In any case, ice, rest, and elevation will speed up recovery.
