I Have Been Experiencing A Lot Of Cramps During My

I have been experiencing a lot of cramps during my period .. But that's not what I wanted to ask.. I just went to the washroom had the feeling I had to pee, but instead I looked and the toilet was full of blood. I have regular periods and they are usually a day or two days early, and lasts for 7 days. I have never been extremely heavy while on my period. I took a pregnancy test before my period and it was negative. I thought I was having a miscarriage, I've also had dime size clots. And I have went through 18 tampons yesterday. Someone please help.