Dear SHYLEEANN: Gail has given some correct information, but I would like to add a couple more things. You mention that sometimes you have clots which are half dollar size. Usually clots that size are linked with a heavy flow. You see, there is an enzyme produced by the lining of the uterus designed to keep menstrual blood from clotting. This allows the menstrual fluid to more readily pass out of the tiny cervical canal and into the vagina. When a woman is bleeding very heavily during her period the amount of flow surpasses the amount of enzyme--thus leading to passage of large clots. Fortunately, the action of this enzyme is localized to the inside of the uterus so it should not have anything to do with inability of other blood to clot.
In truth there is a condition where menstrual blood and lining do get "backed up" inside the uterus. The medical name is hematometria. The most common cause is that something will cause the opening from inside the uterus to scar closed so that blood cannot escape. The woman may miss her flow and have bad cramping. However, this is a condition which would be present every month until the canal is re-opened with an instrument. Also, the retained blood can be easily seen on ultrasound.
My best GUESS about the hip pain around the time of your flow MIGHT be referred pain from the contractions of the uterus. There are nerve pathways which are shared in common between various body sites. For example, some women will feel their labor pains (uterine contractions) in their backs as back pain or down the insides of their thighs.
Much more remotely, pain which is present just during menstruation, MIGHT be endometriosis implants. While endometriosis tends to create pain in the uterus/tubes/ovaries, it can be found in such distant locations such as the bowels, rectum, bladder, and diaphragm.
Lastly, bloating around the time of menstruation can be from either slowed bowel movement (due to the smooth muscle relaxation effects of the hormone progesterone), or fluid shifts (like swelling of fingers, feet). Some women typically gain, then lose, a couple of pounds of water weight during the "PMS time."
I'm really sorry about the hip problem/ pain. That is a terrible burden to have as a younger woman.
Yours,
Jane