Why haven't I had my period?

I'm 19, 5'3, and weigh between 93-97 lbs... haven't checked lately. I eat 1300 calories a day, and only exercise for 30-45 min. 5 days a week (cardio). Anyway, I have a small frame. I've always been thin. So, I don't understand why I haven't had my period in 2 mos (going on 3)?! I have low iron in my blood... could that be the reason? Because I am not that underweight!