What Causes Heavy Periods?

Hi, 2 years ago just after my period had finished I started to bleed heavily all of a sudden. I didn't think much of it and was just waiting for it to stop. It wasn't stopping but was instead getting heavier and heavier. After 5 weeks it got so heavy that I wasn't able to leave the house or to sleep at night. At that point my GP finally prescribed Norethisterone and the bleeding stopped. I had a scan and was diagnosed with a 2,5 cm fibroid. I was advised to stop Microgynone 30 which I was taking for many years and on the second scan 6 months later my fibroid become much smaller, almost completely disappeared. My periods are slightly heavier since I stopped Microgynone and I was taking tranexamic acid which worked perfecty. However now 2 years later just when my period was about to stop I'm all of the sudden have a really heavy bleeding again, really heavy. I don't understand where it's coming from again and don't want to go through the same nightmare again! I'm thinking to start taking Norethisterone again but can not remember the dose and the length of the treatment. The next available GP appointment is in 1 month and I can not wait that long, had to look for toilet on my way from work 4 times and it takes me just half an hour to get home. Don't know what to do and why is it all over again!