What Causes Irregular Menstruation?

I used to get my periods regularly, but for the past 7 months i am not getting my periods on a regular basis. now i only get my periods every 2-3 months. when i don't have my period, my stomach is still paining ocasionally. Another thing, normally my periods lasts 6 days, but this month, my period only lasted for 3 days. Is there something wrong with me?