Many women have clots in their menstrual blood from time to time. The clots may be bright red or dark in color. Often, these clots are shed on the heaviest days of bleeding. The presence of multiple clots in your flow may make your menstrual blood seem thick or denser than usual.
Your body typically releases anticoagulants to keep menstrual blood from clotting as it's being released. But when your period is heavy and blood is being rapidly expelled, there's not enough time for anticoagulants to work. That enables clots to form.
If you have excessive clotting or clots larger than a quarter, you should see your health care provider to rule out any conditions that might be causing an abnormal period.