This depends on where all your endo tissue was, but it may be that not all of it was removed, I mean it depends sometimes if there's some where they may need to remove an organ sometimes they don't etc.
Also, it is a whole body inflammatory condition and so the tissue is always growing and can sustain its self sometimes, also adhesions etc can grow, so 10 months postpartum is sort of enough time and some would have likely grown during your pegenancy as well. It doesn't grow because of your period at all and doesn't have anything to do with that tsther things like estrogen, and prostaglandins help it to be painful.
The main symptom of endometriosis is not actually period pain because endo is not a period condition - it's a whole body inflammatory condition so your pain is felt throughout the month not just when on your period.
It will likely be less tissue but that doesn't necessarily correlate with symtpoms.
In terms of pain an NSAID like naproxen or mefenamic acid can really help, and I'd consider going back on them from now. Also for the bleeding tranexamic acid until you can use something more long-term like the mirena coil. Is the pain quite bad? As you could look into trying some opiods as well, maybe as needed at first? It really depends on the person.
You might need to increase your iron dose if your still really struggling with symtpoms of it. Maybe ask for a prescription dose which can be higher.
Also there are different types of pills you can try so ones which increase and decrease doses across the cycle etc. Ask your GP not just about different types but ones on different mechanisms compared to any you have tried and didn't like.
So during a menstrual cycle, progesterone is released later on (luteal phase) - it basically tells the hypothalamus and pituitary glands, stop now you've done your job (which is thickening the uterus lining - estrogen's job), let's not go overboard and progesterone maintains the uterus lining (whereas after your period, estrogen stimulates it to grow more). Essentially, the hypothalamus, or pituitary glands, at that point stop producing FSH, LH - that means that less estrogen is produced. The mirena uses synthetic progesterone to thin the uterus lining and help to shrink things like fibroids.
It also suppresses estrogen secretion to hopefully stop more growth of the uterus lining (helpful for periods it leads to less bleeding and cramps). That's the pathway for both the pill and the mirena but the mirena is more localised which often means less side effects for most people.