Originally Posted by Eowyn

This is totally second-hand, but I do know that Justin Morneau (who plays for the Minnesota Twins) got a concussion last season, was out for seven months, and then returned to play.

He later experienced a mild relapse of symptoms after diving to catch a ball in the outfield, even though he didn't hit his head directly on anything. I think they reported that he had brain fog and possibly a headache after that. He sat out a couple more games.

So, apparently, yes, you can experience renewed symptoms after minor events.