Iron deficiency is the most common nutritional deficiency in the world, and in women of reproductive age the usual cause is not diet but menstrual blood loss.
The symptoms are easy to attribute elsewhere: tiredness that sleep does not fix, breathlessness on stairs, poor concentration, hair thinning, cold hands. Each has a dozen explanations, which is part of why low iron is often found late.
It is also possible to be iron deficient without being anaemic. Ferritin — the stored form — falls first, and can be low while haemoglobin still reads normal. A doctor testing for this needs to look at both.
Heavy bleeding itself deserves attention rather than just replacement. If periods regularly soak through protection hourly, last more than seven days, or pass large clots, treating the bleeding often treats the iron problem at its source.
Iron from food is absorbed better alongside vitamin C and less well alongside tea, coffee and calcium taken at the same time. Supplements should be taken on advice rather than assumption, because too much iron is its own problem.