Does taking tamoxifen for breast cancer treatment cause cancer of the uterus ?

Yes: Tamoxifen has been shown to increase the risk of uterine cancer, not unlike estrogen replacement therapy, with an incidence of ~1:500. However, this is almost-always caught at its earliest stage, with very high cure rates. If you compare the benefit of tamoxifen for either treating or preventing breast cancer compared to this risk, the benefit far outweighs the risk.

Tamoxifen: If you get vaginal bleeding, you need a biopsy of the endometrium. Some doctors do ultrasounds looking for polips as well