Hi,
Possible causes of your problem are:
 - Reproductive hormonal imbalance leading to inter-menstrual bleeding
 - Side effects of 
hormone replacement therapy
 - Other systemic causes like thyroid hormonal imbalance, blood sugar level fluctuations, coagulation abnormalities, etc. leading to vaginal bleeding
 - Rarely malignancies.
Taking 
mammogram may not be  related to 
inter menstrual bleeding generally. As you had heavy bleeding better to get evaluated once. Please consult your gynecologist, get examined and undergo an 
ultrasound, blood investigations like 
hemoglobin levels, blood sugar levels, coagulation profile, thyroid profile, etc.  With this work-up the possible cause of your problem can be identified and treatment can be planned accordingly.
Take care. Hope I have answered your question. Let me know if I can assist you further.
Regards,
Dr. Sree Gouri S. R., OBGYN