What Causes Irregularities In Menstruation?

Im 25 years old. I have irregular period from past 6 years. A year back I used to get very less periods once in 3 months. But from 9 months, am not getting period. Wen I consulted gyno, some say normal and give me some pills. Once I take that I get periods that s al after dat I don t get. Am worried if I have a major problem as m getting married in 3 months and planning to concieve after a year. Pls tell me wat I should do