I want to lose weight, but Im afraid my fat will turn into bulky muscle?

Ok, so, I'm 13 and i have type one diabetes. A few months ago my blood sugar was under horrible control and I lost tons of weight (not on purpose). Now that my blood sugar is back my body still thinks its being starved, even though I eat ( I eat not that much because i keep gaining weight.) I am gaining weight and I now weigh 140, and I'm only 5 foot 4. Its summer so i want to be confident. I'm trying to lose weight but no matter what it wont come off. I don't eat red meats, or a lot of junk food. Mostly whole grains and vegetables. I do pilates 5 times a week and I ride my bike 2-4 miles also 3-5 times a week. I'm afraid that when i lose the weight it will be bulky muscle on my legs and arms. I just want to be nice and slim. Like toned, but not huge. How can I achieve this? Why wont i lose weight? Any diet tips (plans) and exercise plans? please please please help me!