implantation bleeding?

I started bleeding around the time I was supposed to have my period. But there was NO cramping which is unheard of for me. However, it went on for two days of heavy flow (my tampons were completely saturated when I changed them). Then the next two days there was a slight brown blood color when I wiped. Could this be some sort of heavy implantation bleeding? I just think it's weird that I didn't cramp at all if it was my period.