simple - eat more.
(And, no, 1600 cal is not considered 'a lot' for an active teen who should not be losing any more weight.)
Make sure your water intake is high enough. Fiber without enough water can be constipating.
Drink a lot of water ! ! ! ! !
Also make sure you get enough healthy fats. Nuts are healthy fats but they can be hard on your stomach. A few tablespoons of cold pressed vegetable oils will help to oil your inner organs and make you go easier to the bathroom. I can recommend Udo's Choice, perfect oil blend of healthy fats.
Also, you can eat some raw ginger root. Make tea of it or put it in your smoothie. That will help your bowels as well.
Exercise also makes sure the systems works as it should, so make sure you do some exercise, if you're not already.