Will eating less but not exercising cause me to loose muscle?

I've been having digestion issues for months that have caused me to have no energy at all. I hadnt been eating enough so my dad insisted i eat more. When i ate 1,500-1,600 calories a day, i started GAINING weight! I'm not exercising, but still as a teenager at 14, shouldnt i be able to maintain my weight with that? also has it caused me to loose muscle? i've gained like 3-4 lbs and i appear flabbier, but idk if thats fat gain or muscle loss. According to a calculator, my bmr is like 1,300 something and the reccommended calorie intake for sedentary activity is like 1,600-1,700 but idk how accurate that is for me since i'm a teen. I do have a slow metabolism, but doesnt it seem unreasonable for it to be THAT slow? Also im not sure if walking from cl to cl with a pretty heavy backpack on would count as sedentary either....sometimes i dance around really crazy for like a half hour out of boredom but i cant really count that as daily activity. I am a former very active person. I used to work out 6 days a week with weights and cardio but i have been feeling so bad, and my dad thinks the digestion issues were caused from not enough food and slowing my bodily processes. I'm confused.... help?