Turns out that it's an enormous clot, and my uterus is contracting, to try to pass it. My OB did a scan, and the size of the clot appeared to be about the size of my pre-pregnancy uterus. He's given me scripts for 2 types of antibiotics, and I'm to go and see him again in a week to be reviewed.

I'm relieved that it's nothing serious, but it's also good to know there is a reason for what I was experiencing.