If you have been having sex, it could be a miscarriage. The lack of breast soreness doesn't particularly suggest it, however. It could be leftover tissue from an earlier (very early) miscarriage. It is also possible that you had a decidual cast.
Well, yes, we have to ask if you've been sexual active. Some women have chemical pregnancies which means they were pregnant very early and naturally miscarry. And it is like a heavy, more painful period. But you'd have had to have sex for that. Another possibility is a cyst. Women get those from time to time. And when they pass, it's painful. Most women will pass a simple cyst without any issue other than what feels like a heavier and more painful period. If you continue to have issues, I'd look into endometriosis or fibroids as well. When is the last time you had a pap smear? That's important to do and will give insight to if there are any problems.