What Causes Heavy Bleeding After An Abortion?

I just had an abortion and I am bleeding worse than the day I had it. I also have pain in my left side by my uterus. I am feeling a bit dizzy and when I stand I sometimes loose the ability to see for a couple of seconds, and shortness of breath every once in a while. I have von Willebrand disease but other than that I am pretty healthy. I usually have normal periods. Should I be worried?