need advice as to what's going on!

I'm 19 years old I haven't been on birth control in over two years, I have a regular period every month around the 20-25th . Last month I started on June 19th lasted about 6-7 days so it was kind of early..July 2 I started bleeding out of nowhere and now it's July 5th and I'm still bleeding off and on sometimes it's blood red and sometimes brown it's not just spotting either..I have been having some cramping as well..I took a pregnancy test this morning it came back negative..what could this be? I've never once in 5 years had this happen to me.