Scared hysteroscopy/biopsy on 3/12 no hyperplasia/cancer put on prometrium (micronized progesterone) 2 months ago day 6 cycle 6mm this one day 4 is 12 mm how likely cancer now& why meds not working?

Very low: With a negative biopsy for both hyperplasia and cancer very unlikely that you have cancer despite the continued symptoms. However, it is not impossible. There is a small percentage chance perhaps 2-4% that a small cancer or hyperplasia was missed. Make sure you have follow up and consider doing a repeat ultrasound in six months as well as a biopsy if still bleeding to assess the uterine lining.

Very very unlikely: Lining in a premenopausal patient us normal up to ~25 mm at certain phases of cycle. The 5mm rule only applies to postmenopausal patients. You appear to not be obese. Smoking and diabetes are also risk factors.