Miranda, it's great that you are so active, have cut out the junk food, and are eating so many fruits & veggies! That is fantastic. It is very possible you've had a recent growth spurt since your last doctor's appointment. I have this hunch you may have grown another inch, which would explain the unexpected slimming down. Check your height!
As a swimmer & a runner, you're going to need lots of carbs. Choose healthy carbs rather than junk food. Stay away from fad diets that require you to cut out entire categories of food. This includes gluten free diets (the latest fad)-- Celiac is real but rare, and only an endoscope biopsy can prove you have it. If you are looking for a reliable source on nutrition, google Dr Walter Willett Harvard University or check out this site: http://www.hsph.harvard.edu/nutritionsource/
Your brain uses 70% of the carbs you take in every day. And your brain right now is growing/ pruning at an enormous rate (read the book: The Primal Teen). So you should only be cutting carbs if you are diabetic.
In a nutshell, here are some guidelines:
1) Strive for at least 9 servings of fruits & veggies a day.
2) Have some healthy fats at every meal: very important because many veggies have fat-soluble vitamins so you need a bit of fat or otherwise you're not getting the full value.
3) Have some protein at every meal, but also realize that the body cannot process more than 6 oz of protein a day. I don't care what the body builders say.
4) If you can tolerate it, it doesn't hurt to have a bit of dairy at each meal (yogurt, milk, cheese).
5) Have a handful of walnuts every day.
6) Make up the rest of your caloric needs with healthy carbs - whole grains. Increase or decrease the servings of carbs according to your exercise needs that day. So on days you swim or run, you can have an extra serving or two of carbs. Protip: I always find if I cut my carbs too low, I have problems falling asleep.
7) Minimize process foods, refined grains, junk foods. But at your age and weight, you can probably tolerate a little -- say, a serving of junk a day.
8) No sodas, not even diet sodas.
9) Fruit juice in moderation (one small glass a day). Or not at all - have whole fruit instead.
The most important thing you can do for your body is to nourish it and exercise, while aiming to maintain your weight. Dips up and down at an early age put one on a course for a lifetime of yo-yoing. Research shows that this yo-yoing is more harmful than maintaining a consistent weight year after year (no matter whether that weight is under/over or at ideal).
