What Causes Pain In Abdomen During Menstruation Cycle?

I am 25 years old. I have a regular period but with very heavy flow. I feel pain in my lower abdomen during the 2nd and 3rd day of the cycle as well as I feel tired and cannot do heavy work. Suddenly while having my last period large blood clots were excreted on my 3rd day. I am not taking any medicine. I want to know why this is happening and what should I do to reduce the pain and fatigue. Can it cause infertility?