What Causes Pain During Menstrual Cycle?

My last menstrual cycle was extremely painful. The only thing i could describe it was that it felt like i was in Labor all over again. My youngest child is 5 1/2 months old now. I was seen at the ER for the painful cramps and they prescribed me percocet but I'm worried that the cramping will be just as bad with my next cycle. My husband has noticed lately that ive been more irritable and ive noticed that ive been more fatigued and achey since. Is this something that could be serious?