Possible miscarriage?

I am 2 weeks early for my period. I have bad back pains, and I just started bleeding? I am concerned that I am having a miscarriage.. My period is always normal & happens like clockwork! I took a pregnancy test & it came back neg. I would have just thought that I was having an early period., but all I am having is just blood clots & lots of them. Has this happened to anyone else?