How can I tell if I miscarried? Very concerned what is happening with my body..

So, me and my boyfriend had unprotected sex. About a week later I went on birth control. 
Its been a little under a month since the occurrence and out of nowhere, I got these horrible stomach pains. That later turns into my lower back feeling like someone is pinching every nerve and then my knees did the same thing. Because of the pain, I got sick to my stomach and am throwing up stuff that should have already been digested. And yes, it was coming out the back too. The next day its just severe cramping in my gut/back/knees. Then I notice that I started bleeding a moderate flow that gets lighter at some points and heavier in the next throughout the day. Which I shouldn't be one because it is way out of the way of my cycle and two I have been on the pill (I wasnt in the inactive pills yet). I did notice some clotting come out onto my pad, too. I've only had cramps like this once before but that was over 6 years ago and they weren't this bad; they only lasted a day. Plus they went away after I started bleeding.
I have no idea what is going on. A couple weeks ago, everything I ate made me sick and I always had cravings. I just thought it was the birth control, but now I'm just not sure.