I think if you eat an appropriate amount of nutritious food and move your body some everyday, you are just fine. Your weight will fluctuate in the early stages on T, but also I have to note that at your age (regardless of transition) your body is likely still growing and changing. Some of what you are going through is just normal human growth and development that evens out over time. It really is important, though, to include moderate activity in your daily routine and to eat nutritious food when you feel hungry. Because of the over-emphasis on obesity here in the US, all of the height/weight/BMI numbers can be deceiving, so I would not put a lot of stock into what the scale says. Optimum health is not strictly based on height and weight. You might learn more about your picture of health from your blood chemistry, your emotions, and your daily energy level.
I am technically borderline UNDERweight, with huge muscular legs, gangly arms, and a good sized gut that hangs out a bit over my pants. All bodies look different...try to gauge your state of health from within!