Look at my username - I've experienced menorrhagia for the last year or so on and off. No known cause for mine, but I think one of the likely causes is the covid vaccine - and yes, 6 months after the jab it's still possible that that's a factor for you, although sadly I've yet to find any health professional who will actually talk about and acknowledge it, there IS decent quality research showing a link and a fuck ton of women who are reporting anecdotal evidence.
GPs remarkably unworried about mine, (although to be fair my GPs were utterly shit and I put in a complaint about them on this issue), so I doubt A&E would do anything... however I think you perhaps should still go on the basis that you can't rule out miscarriage (I could).
Traxamaemic acid could still help you this bleed, although it takes a few days to kick in.
You'll want a GP to refer you for an ultrasound to check for endometritis or fibroids.
Don't get fobbed off by being given hormonal drugs or being told to go on the coil or birth control - they may stop or significantly lessen the bleeding but they don't address whatever the cause is.
Weirdly a lot of us have success with taking ginger - some take it as tablets of ginger powder ordered from Amazon, I have had success with ginger shot drinks (Lidl sell them in little cans, Sainsburys sells a 500ml bottle for around £5, or just get fresh ginger and slice/grate in water, steeped as tea or just in cold water, both works) to reduce the flow. I also find that eating plants with phytoestrogens throughout the month helps mine - that's edamame beans, tofu and ground flax seeds.
Your iron levels could still be too low or too high, as the standard iron blood test isn't actually very helpful - Either one can cause heavier bleeds, so unless you have reason to believe it is too high try getting more iron and vitamin C, in your diet or supplemented or both.
Eat as if you are diabetic - don't go Keto or Atkins, but sensibly low on complex carbs and avoid simple carbs and sugar as much as you can without being silly about it - I try to stick to less than 30g a day added sugar, which allows for some chocolate.
There is a group on facebook that I found REALLY helpful - Menorrhagia & Heavy Periods & Endometriosis Sufferers. Go and ask for help on there.
They can also recommend what to wear with the heavy bleeds. Personally I found post partum pads most useful; others use full incontinence knickers (incontinence pads have way higher absorbancy than menstrual ones).