Knee Pain?!?!?!?!?!?!?!?!?!?!?!!?!?…

I rode a exercise bike yesterday for 8.5 miles. When I woke up this morning I could barely put my leg out straight and it really hurts when I put pressure on it. The pain is coming from the backside of my leg right behind my knee. In highschool my doctor told me that I have a floating patella. I dont know if I just over extended my leg or if I need to go to the doctor to get it looked at.