I had something similar happen to me in Boston: One minute, I was starting up after the light turned green, the next minute I was flat on my back four blocks away with my face all torn up from a close encounter of the asphalt kind. I later deduced from my completely bent front wheel and the lack of any nearby road obstacle that a car must have pulled out in front of me. They took me to the ER and did a scan, and all was well - no concussion, no nothing. So definitely get looked at, but don't assume you have a concussion or had a seizure necessarily.