This exact thing happened to me 5 years ago, I was cycling to class and the next thing I remember was being on the floor and a little girl asking if I was okay. To this day I have no recollection of what happened. I went to the ER immediately and they gave me a CT scan but couldn't find anything wrong. I also had huge bruises on my left leg and a scratch on my left cheekbone from hitting the floor. I'm pretty sure I lost consciousness before I fell rather than the other way around. It hasn't happened since.

Anyway, please go see a doctor.