What Causes Irregular And Prolonged Cycles?

Hi Im a twenty year old college student, 157lbs, 5'4 and Im also virgin. Not a sickly person only experienced childhood asthma. I have always had irregular periods but my cycle comes about 4 times a year while also having severe headaches and feeling lightheaded. When my cycle finally comes it comes and goes for about 1month and I lose an extremely large amount of blood. What should I do?