I had gastric byp in 2006, why 3 yrs later am I just starting to have trouble with iron deficiency anemia?

Within the past month I went from a 10.0 hemoglobin level to a 7.4 level and was admitted to the hospital and had to get 2 blood transfusions. I have not changed any of my eating or daily activities these past 3 years. I am trying to find out why it has taken this long to start having trouble with my iron? The doctor at the hospital said I will not be able to take iron pills it would do no good and said I will have to have iron iv injections to keep from my hemoglobin from getting so low and getting me put back in the hospital needing more blood transfusions. Someone help me with this...have you had the same experience years after having the surgery and is it true that taking the iron supplements by mouth will not work?