Breakthrough bleeding on HRT

I was fully in menopause and began HRT. Was still experiencing night sweats so Dr. upped the estrogen dose (kept progesterone same) multiple times over months as night sweats continued. Changed medications multiple times. And now, 2+ years later, I’ve started having breakthrough bleeding that lasts sometimes one week, sometimes, two weeks. Had an ultrasound and all seems fine but bleeding continues every three weeks or so. Dr. Said sometimes breakthrough bleeding can occur on HRT but it should level off quickly. But, this has been going on for the last year or so. What is happening?