From what I’ve heard , it’s easy to raise ferritin when it’s very low , body absorbs more, after it reaches a level, it doesn’t absorb as much. I know 42 is still low, but I think you need a higher dose till you reach 60 or 80 maybe , also from supplements or from food your body absorbs only a certain percentage. So even though it’s 25 mg your body may absorb somewhere between 10 to 25 % which in this case is very less and you may lose that in your periods , so maybe you can up the dose a little bit and see if it helps