Yes, I had this! This is from the tamoxifen, and why I decided to have a complete hysterectomy, including my ovaries removed. It has almost been a year now, and I don't regret this! I actually wish I would have done it sooner because it solved the problem of birth control, and I now feel closer to my husband again.

Within a couple of months of starting Tamoxifen I had painful, heavy periods. When I finally had a vaginal US after 6 months of complaints = thickened endometrial lining. My GYN did a D&C because her thinking was that an endometrial biopsy is very painful and in too many cases comes back suspicious and end up with a D&C.
I haven't had painful periods since the D&C in May, had regular periods for a couple of months and now had my last (hopefully for good) period starting 8/25.

My MO did warn me to have them check for thickening of the uterus, but I do not know the exact why of it. Good luck

I have been spotting, today had a biopsy. My gyn saw a mass in my last MRI of the pelvis. Her face said a lot. I have been in tamoxifen for a year also

I would be concerned too. The drug does increase risk of uterine cancer. Definitely take it seriously and have it checked out. I have not yet taken Taximofen, but my oncologist talked to me about it, since it is a part of my treatment.