You may have fibroid tumors. These are benign (non- cancerous) growths that happen in your uterus. They can cause the symptoms that you are having with the pain and clots.
Here is some general inforamtion about them:
http://www.fibroidfacts.com/Home.do
There are many treatments for fibroids including hormonal medications and procedures that cut off the blood supply of the fibroids, all the way up to hysterectomy.
The first step is to get a diagnosis. Your gyn will send you for a pelvic ultrasound to look and see if this is the problem.
The pain can be a major problem with fibroids. Also of concern is excessive blood loss during your periods due to excessive bleeding. Your GYN may want you to have blood work to assess your blood count .
Please let me know if you have any other questions!
Take Good Care!! Nancy