You may want to try consulting with an endocrinologist. This is the type of doctor that would diagnose a problem with your hormonal system. I think it would be worth while to rule out any type of thyroid problem because these are the most reasons why people will experience changes in their metabolism.
At such a young age I would not anticipate you having such difficulty maintaining weight particularly given how much activity you do.
Also consider other aspects of your diet. It is great that you have cut out soda’s and are eating more fruits and vegetables but also watch how much carbohydrates you are taking in. They will make it very difficult to loose unwanted weight.