20yrs ago, I had a ferritin of 3 and Hb of 9. I was getting UTI's, knackered, dizzy and unusual food cravings. I now know food cravings can be a sign. It sounds ridiculous now, but I would crunch on ice cubes. I even drove through mcdonalds- just for a cup of ice! 🙄I was clubbing/uni/semi professional dancer and just not eating well. No bleeding issues.
I was offered iron injections- not IV, but into the muscle. I declined, because unless they use a Z track technique, it can leave a permanent tattoo in the skin! I don't think an injection into the muscle is done now. Some places will do an infusion, but I don't know what the NHS recommends now. Ferritin is your iron stores.
I took the prescribed tablets and drastically changed by diet. Red blood cells do take 3mths to replicate, but I started felt better in a few weeks. Take your tablets with Vic C rich foods- orange juice, peppers, kiwis and avoid food rich in tannins. I increased by red meat and started eating liver. With bacon and onion gravy, it helped disguise the flavour and I actually like it now.
Whatever you do, don't change to spatone/floradix or over the counter supplements. These are for maintenance only and not strong enough to correct such a deficiency. A bit like saying you can fill a swimming pool using 1 teaspoon of water a day! A blue/green algae called spirulina is a supplement which can also help increase your iron, but again, NOT to replace the prescribed meds you get.
Edited to say, if your periods are so heavy, I'm surprised your GP hasn't given you tranexamic acid! Have they suggested going on the pill or other contraception which stops your periods?