help!!!

So my period was about a week late, and then I finally got it last Sunday. By Monday morning, it was completely gone. Tuesday morning it decided to come back, and then by Tuesday night it was gone again. When I did have it, it was really light. Before, during my period, and still I'm having really bad cramping. I took a pregnancy test the day before I got my period, and it was negative. It's worrying me that something is wrong, because my period always lasts at least 5 days, usually 6. I also think it's weird I'm still experiencing cramping. My boyfriend and I have unprotected sex every time, but I got a negative pregnancy test. Any ideas as to what this could be?