As I am sure you know, given your very serious history, any clots larger than a quarter should be reported to your doctor.
Have you been tested for bleeding disorders such as Von Willebrans Disease? Excessive mensrual bleeding is one of the #1 signs of this disease in women but it isn't a test that is typically run in a normal panel of blood work. Maybe mention running tests for bleeding disorders to your Gyn, or maybe even contact a Hematologist for a consultation.
Good Luck to you!
--J
It could be just uterus lining buildup, but I would call your doctor.