Like akelley23 said, you aren't eating enough. Unfortunately, a lot of people are under the impression that they will only lose weight if they eat the bare minimum and exercise a lot. Exercising as much as you do and eating as little as you are is going to make your body hold onto anything it can, and anything you lose would come back once you returned to a sustainable lifestyle.
Find out your BMR: I've been using this site to calculate mine - http://www.bmi-calculator.net/bmr-calculator/
Don't eat below your BMR. Make sure you eat more depending on exercise.
Hope this helps.
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