Go see a doctor. Absence of period for 3 months with no pregnancy is not normal especially if you didn't have this problem before...Have you taken other preg tests? If you are sure about the tests then it is somthing else.
Theres a possibility of it being something called PCOS (polycystic ovarian syndrome). Its not really a common thing but if you are having symptoms such as facial hair increase, increase in acne, huge increase in weight in a short amount of time, or depression (which is an uncommon symptom) you should really go see your doc to get some blood work done to also check your hormones cause thats also a symptom. So basically to sum it all up just go see your doc and get everything checked out just to be on the safe side.