Period or pregnancy

I started my "period" super early like two weeks early. I don't know why either. I've had an IUD for a year now and it didn't effect my cycle at all. My "period" started out like normal but it's a lot lighter than normal like I'm only using regular tampons and I'm a girl that goes through like 5 super plus tampons a day. I took a pegnacy test and it came back negative. What could this be? Could it really be my period this early?