What Causes Delayed Menstruation?

hiii.. my periode is late over 3 months now, im 18 years old and im still a vergin, so im sure im not pregnant.. i have a periode pain but no blood , i went to a doctor he gived me one medication and said that i should have it for 5 days than i get my periode in 3 days but nothing happend . what's wrong with me HEEEEEEELP PLZZ im freaken out!!