Why am I gaining weight when I have dieting and exercising?

Anonymous
I've been doing well with my diet (I'm 14, 124lbs, 5'2.1) (eating 1300-1400 cal a day, exercising nearly everyday.)
i lost 3 pounds and I was happy. I couldn't wait to lose more. But then I started to gain it back and I'm gaining more.

Im so confused, because I'm eating well. Every 3 hours, no junk food, 4-5 bottles of water, eating probiotics, greens, protein,...

im also exercising a lot. Today i walked 15,000 steps.
I have bad PMS so I gain period weight easily and I think it could be that but it's been happening for a week and a half. My period starts next week.
Why am I gaining weight when I have dieting and exercising?
3 Opinion