What is this blister?

This blister appeared on my right knee after a mountain bike ride. I do not know if its a bug bite or what. It does not itch, there is no pain. It just is there. I took a picture and was hoping that someone could shed some possibilities. Most importantly - does this seem to be anything I should go see a doctor for? Remember, I do not know its even there until I look at it. It started smaller and grew to this size. It has been this size now for about 36 hours. Its about half inch or less in diameter and protrudes by about 3/16 inch from the surface of the skin, maybe more. I think its filled with fluid.

Thanks in advance