Iron deficiency?

Im 15 and have had some problems with my iron, its been too low and I don't know why? I eat red meats and greens and I don't have heavy periods infact they've stopped all together for about 4-5 months and no im only 15 im obvs not pregnant! I've been taking iron tablets given from my doctor which haven't worked because I got another blood test done and its still low, I feel so exhausted and dizzy all the time and just don't know why my iron is so low. Anyone???