I think I had a miscarriage, but I'm not sure. My symptoms include back pain, a very heavy period, that came after what looked to be a very dark blood clot which came out while urinating. The last time I had unprotected sex was about one month ago. Could this have been a miscarriage?

It's possible, but it's also possible that you're just having a heavy period. It happens.

There's really only one way to know if this is a miscarriage, and that's to see your doctor.