Why would my doctor order an endometrial biopsy? is that a cancer test?

May or may not be: Endometrial is the biopsy of lining of uterus done with a catheter usually in your gynecologist office ( v/s D&C ) not only just for cancer , it is for other conditions, like abnormal, bleeding & pap smear, for thickened lining or polyps, to evaluate uterine response to hormonal therapy, in infertility, and is relatively painless safe procedure.

Can detect cancer: It is not necessarily a cancer test, though the biopsy can detect cancer. It may also be ordered to diagnose infection, endometrial hyperplasia, endometrial polyp, anovulatory state, hormonal disturbances and in general to evaluate abnormal bleeding.

Yes: If by us endometrium is too thick it can mean cancer and biopsy may be appropriate.