What Causes Irregular Menstrual Bleeding?

Ive always been getting my menstrual cycle around the same time ever since i was 12 i am now 21.. ive been intiment with one guy, since i was 18. My last cycle i only bled every now and then. Not even close to getting a pad stained though? What could be going on?.. i got my period july5th now its aug the 9th