Not sure what to make of sons behaviour

Our 3 y/o son has started displaying some strange behaviours as of late and my husband and I are starting to become concerned. It started after what we were told was a juvenial migrane (migraine) 2 months ago and is progressively getting worse. After his migrane (migraine) we took him to the hospital because he was muddled and not making sense. We were rushed straight thru because his right pupil was dialated and the left was normal. The doctor looking after my son did not seem to concerned by this, but the nurses were worried about it. Ever since he has been showing slightly obsessive traits. It started with compulsive hand washing, which he still sometimes does, but he has move onto checking the mail box all the time sometimes 10 - 15 times a day and is not satisfied by just popping his head aroud to see if there is any mail, he has to walk down to it and check inside. If he does not do this he turns around and starts again to make sure he's checked inside. He also has an extremely vivid imagination, but finds it hard to recall somethings done through the day. I know this imagination is age appropriate, but it is starting to concern me because he believes the stories he is telling and becomes distressed if anyone tells him otherwise.
I may just be being a neurotic mother, but I have 2 other children and have never come across this. Is this normal of should I get him check out further?