I've always heard that the helmet's interior structure is like the crumple zone of a car--it will deform once to absorb impact, and then it's no longer effective (or at least no longer as effective). So you should probably replace the helmet, or at least make sure you land on a different part of your head in the next crash.
(I've been reasonably successful with the latter option, but I don't recommend it as a long-term solution.)
And re: concussions, yeah, if you feel a headache, you've probably got a concussion. Did anyone see you crash? It's not at all uncommon to have what you thought was a minor fall and then realize that you don't know how your friends got from 50 yards away to immediately around you in the three seconds after impact, until someone points out that you were unconscious for thirty seconds or a minute. If no one saw you, you may have actually been out for a brief period of time, which would be a definite sign of significant impact to the brain.
If you have any other signs of concussion, you probably should consider consulting a doctor. I can recall at least one teammate who was forced to sit out of training and racing for a matter of weeks due to a minor concussion; the concern from the medical staff was that he was at extremely increased risk in the event of another impact. And the concussion occurred when he caught an edge during an inspection run, while wearing a helmet.
:wink:
