Heavy abnormal bleeding 11 days before period

Hailey
Hey, so yesterday I started bleeding (it was a dark brownish type color which is only usual for me when I draw close to the end of my mental cycle) however it's progressively gotten worse as of today (for example I've went through three overnight pads in an hour) this is extreamly adnormal for me. I usually have very light periods that only last four days max; and let's not forget to keep in mind that I'm suppose to start my period in eleven days. 
My main worry is because I'm anemic and become very weak and sick, sometimes up to the point that I have to be hospitalized when I'm on my period. 
Also another factor that worries me is I had sex with my boyfriend about a month ago. I wasn't aware at the time, just how many girls he had been with prior to being with me. Let's just say the number isn't small. But anyways I just would very much so appreciate all of the advice I can get and I would like to know if I should be worried about it possibly being something linked to having sex.?