M.P.
I suggest that you're borrowing trouble. Your doctor says she's OK. Trust your doctor. You can get a list of possible causes for low iron but they won't apply to your baby. You could scare yourself and for what good?
Because you are worried, I urge you to call her doctor and discuss this some more. Tell him you're frightened and want to know why he accepts her level as acceptable. I assume he told you to continue with the iron supplements and iron enriched foods.