Iron-deficiency anemia

I'm not sure if this is the right place to post this, but here goes....  Back in February 2012 I went to my doctor for a routine follow-up on my BP and some medication changes.  I had been very tired and was having some menstrual "irregularities" so I menitoned that I wondered if I could be anemic.  She thought that it was plausible and ran some bloodwork.  My iron was 74ug/dl despite being on a multi-vitamin with iron.  She suggested adding an iron supplement, so I did.  A couple of weeks ago she ran some additional b/w and my iron had fallen to 51ug/dl even with the iron supplements.  Technically, I'm NOT anemic, but if it falls much more, I will be.  What can cause this?  (BTW, I'm 44, have hypothyroidism, 2 clotting disorders, have asthma, have high BP, and am not yet menopausal.)