zeg--Welcome to the boards!
When you have heavy periods, clotting can also happen. All blood contains a clotting factor. To allow the menstrual blood to flow freely via the uterus and leave your body without clotting, the uterus produces an "anti-clotting agent." But.... when the menstrual blood loss is too heavy, the "anti-clotting agent" can be used up before a period has finished, so that the remaining blood is likely to form clots. Hope this helps. FLFG