my iron levels are falling and i don't know why?

Hi. There could be lots of reasons. 

Low B12 and/or Folate

coeliac / gluten / ibs stopping absorption of iron

vegetarianism and/or lack of vitamin c

a bleed in the body like the bowel etc 

realistically you should consult your doctor on supplementation and aim to get your iron stores (ferritin) much higher say middle of the range.  If you haven't already get your b12 and folate tested. 

Good luck 

Your iron levels can be low just from diet alone.  It does not have to be from any kind of bleeding from the body.  Low iron levels are actually very common.  Although you would not be considered anemic yet, because your hemoglobin and RBC is normal, it can still be benefical to take an iron pill, to help avoid anemia or even a muti vitamin with iron in it already.

Simple anemic workup such as a stool sample would find any bleeding through bowels caused by chronic bleeding from ulcers ect. Lots can cause low iron. It's kinda odd that your hemoglobin would be normal, if your iron is low, being that iron is used to make hemoglobin. Im guessing your platelets were fine, from the cbc test? Other reasons for low iron are not eating enough of it, the body isn't absorbing it such as celiac disease, ect. I hope they find out please let us know.

Really, you can't be diagnosed with Iron deficiency without anemia either, as your ferritin levels are still in the normal range and with your hemoglobin in the normal range, you cannot be diagnosed with iron deficiency with anemia.  I will tell you this.  I get anemic on and off.  Sometimes my iron and hemoglobin will be low the same and sometimes, just my iron will be low but the hemoglobin normal.  I asked my doctor about why my iron was low but hemoglobin normal, as I was scared because of the causes I read on the computer, it says anemia of chronic disease is the most common.  Anyway, he told me to just take an iron pill daily and that low iron with normal hemoglobin is normal before turning into iron deficiency anemia because it had to do with the life span of the RBC which is like 120 days.  So it can take some time for the hemoglobin level to "catch up" with the iron levels.  Of course, I can always be wrong.  I am not a doctor, I just wanted to share what my doctor told me.