I feel like h had miscarriage but beta HCG is...

Miscarriage and negative B-HCG ?
Well, there is no chance that miscarriage happens without confriming the presence of pregnancy which is done by measuring the B-HCG level in the blood (the pregnancy hormone). Considering the BHCG is negative, it means there is no pregnancy and then there will be no miscarriage.

Treatment of heavy menstruation:
You should see a gynecologist to do a full examination and determine the cause behind your heavy menstruation, as the treatment depends on the cause. There are different types of treatments that can be used such as are Tranexemic acid, daflon, birth control pills and progestin pills that  help correct hormonal imbalances, and sometimes the IUD may be used, which reduces blood flow Menstruation.

Diagnosis:
Clinical medical examination at the obstetrician gynecologist, a complete blood test to detect anemia or low platelets, thyroid hormones TSH testing, pelvic ultrasound, cervical Pap test, and endometriosis biopsy can also be done. Hysteroscopy

Causes of heavy menstruation:

• Hormonal Imbalance
• PCOS
• Uterine fibroid
• Polyps in the womb
• Using an IUD
• Some blood diseases such as von Willebrand and others
• cancer
• other reasons

Recommendation:
It's recommended to visit the obstetrician and gynecologist for further assessment and management.