What Causes Heavy Period With Severe Abdominal Cramps?

I have a delayed period for 9 days. The next day, my period arrives with bright red blood and many clots. My lower abdominal is very painful. It's my first time to have these since all my life I have a normal menstruation. But I have an unprotected sex few weeks ago during my ovulation. My lower cramps is different from normal menstruation and it last for 24days. What does it mean? Could it be a chemical pregnancy? I also feel my breasts sore but not my nipples and I easily feel fatigue and little cramps to my lower back.