Take another test to be sure, then go in for an exam at the Dr's. Your cycle will change numerous times during your life for no reason. However, weight, stress, habits (both eating and exercising), life changes, etc will all have some effect on your cycle.
You may also want to talk to the Dr about bc options. At 19, if you don't want to become pregnant, I'd advise against the withdraw method. You may be very in tune with your body and know when you are ovulating, etc., but at that age, I wouldn't be taking the risk.