Mom with menopause had periods of heavy bleeding due to which she is put on hrt. endo biopsy negative. are these episodes of heavy bleeding normal?

No, not really: Heavy post menopausal bleeding is never normal, as it can be a sign of uterine cancer. She's had a negative endometrial biopsy though. She might have benign uterine fibroids (uterine nodules) or polyps. They can cause bleeding. Hopefully she's had a CT scan or pelvic ultrasound scan. If the med treatments don't work, then she could consider a hysterectomy or a uterine ablation procedure.