A menstrual period question?

When a teenager (17), should a girl be worried about an irregular period? Details: started period at 11, the problems started at around 13-1/2, not ually active at all, exercise daily. If the period is absent for more than a month, then reappears for 3 weeks and is very heavy, should she be worried? What about a light period that lasts 10-14 days? I heard a girl's period will be irregular for a the first few years, then be regular after about 14 or 15, but should it be THIS irregular this long?