What Causes Abdominal Pain And Blood Clot Discharge After Discontinuing Birth Control?

Hi,
I have had heavy bleeding for over 35 days. I have been passing large blood clots and have had constant pain in my lower abdomen. There are times when the pain is so bad that I can barely move. I assumed the bleeding started from me stopping my birth control. However, the symptoms continue to get worse. I went to the doctor today and they scheduled me for a pelvic ultrasound exam. I am now to the point where I cannot handle the pain and I am getting concerned. Do you know what may be causing this? Or what might be wrong with me?
Thank you!