I'll take a chance - it was prob a one off event. Couldn't be serious when it happened four weeks after my accident, especially when I was kept for nearly two weeks in hospital for 'observation' (they are really milking tax money), and bearing in mind all the side effects of the injury are all but gone - no headaches, or the confusion that I suffered for the first couple of days.
Don't let your pride get in the way of common sense. Head injuries are not something to play with. One of the reasons they told you not to work or drink I'm sure, was in case of dizzyness. Don't take a chance with your life. At least call them and tell them.