Serum iron level is about 9-27 (depending on the labs set values), Hemoglobin is about 12-16 for women. Here are the symptoms of anemia: "easy fatigability, increased heart rate, palpitations and rapid breathing on exertion. Severe deficiency causes skin and mucosal changes, including a smooth tongue, brittle nails, and cheilosis (red lips and fissures). Difficulty swollowing because of the formation of esophageal webs (Plummer–Vinson syndrome) also occurs. Many iron-deficient patients develop pica, craving for specific foods (ice chips, etc) often not rich in iron."
If you were bleeding the hematocrit would be low and you would either have blood (bright red or dark depending on location) in your fecal matter or blood in your vomit (bright red or coffee ground appearance depending on location). It's most likely iron deficiency. Vit B12 will be differentiated from iron deficiency anemia via the normal CBC that the Dr would've ordered, which would usually show an elevated size of the red blood cell (MCV 110-140), iron deficiency is usually nl MCV. Also with Vit B12 deficiency you would have nausea, weakness, vomiting, sore tongue, diarrhea, and paresthesia = "pins and needles."
I've had the same thing where my menstrual flow was scant and dark (Too much info but...) and they found I was borderline anemic. Once I started on birth control pills it corrected the flow problem (I was only on it for a few months), I only took the iron pills for about 3 months. However, you should be on it for about 6 months. If you can't tolerate the gastrointestinal side effects and it's been over a yr (about) then ask about an injection (side effect I think is less than 1% of people- Mild hypotension, tightness of chest, wheezing; the other drug has lots of side effects) and let the Dr know that you can't tolerate the pills they will be able to provide you with an alternative.
Hope this helps.