Why am I gaining instead of losing?

I got this app on my iPhone about with the intent of losing about 7 pounds by the end of July. It told me to eat no more than 1, 500 calories a day, and I could track everything, etc, etc.
Anyway, I have been doing that, as well as I enter my exercise for the day, so I'm burning calories as well. It said I should be losing a few oz a day at least.
I weigh myself every monday and I started out at 116.4. Every time I weigh I have been about a pound more! This past monday I was 119!
I'm eating healthier and doing more exercise than I really have in my life (comparison to before, I didn't do ANY exercise and I just ate whatever). I have also always drank a ridiculous amount of water everyday (about a gallon usually).
I would sort of understand if my weight loss just plateaued… Maybe my body won't let me lose anymore. But I keep gaining?
I just don't get it! It would have been a lot easier to go down from 116 to 110 in four months, but now I have to go down from 119 to 110 in three?
Someone please explain? Scientifically and logically this just does not make sense to me.
If you need my stats, I'm 5'2, and 21 years old. My mother and father were short as well but also thin.

It might just be muscle

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