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.
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.