At this point, you should see your gynecologist for an exam. There are a couple of things that this could be--dysmenorrhea (cramps) due to a small cervical opening or even cervical stenosis (narrowing) could cause the larger clots and blood to back up because it doesn't flow freely. This could cause cramps as it comes at at the end of your cycle. It could also be endometriosis, especially with the heavy period and clots, and some of the tissue hangs around and irritates. Adenomyosis can cause an inflammation of the uterine muscles that can cause that kind of pain. You should see your doctor to get a proper diagnosis, and make sure you keep track and are very specific about exactly where, when and what type of pain, as this will help to make the diagnosis and get you some relief.
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Lori