ANDY'S ANSWER: I'm not a medical professional, but I know that periods are a stressful time for a lot of girls, so I would not be surprised at those kinds of symptoms. You have already consulted a doctor which is what I would have recommended you do to ease your concern.
Since her doctor has said there is nothing to worry about, I would say that there probably isn't a problem. The doctor has also said that if it continues to happen to bring her back and I'm sure that is because at that point in time they'll do a thorough check up and a few tests just to see if her iron levels, etc. are good, just on the offhand chance that there might be other things that need looking into.