Sick and unable to eat in the summer?!!!?

For the past two years, during the season change from spring to summer, i lose all appetite, and every time i attempt to eat, i am disgusted with myself. I am only able to eat certain foods, such as pasta. I had a bowl of honey nut chex this morning, and puked from what seemed like "over eating" to the point that there was no room for the food i was consuming. This all of a sudden just hits, and i feel gross every time i eat, so during this period i don't eat whatsoever. I tend to drop a ton of weight, not just including fat but also some muscle, although becoming even more active on top of my already active lifestyle. I am a competitive gymnast, cheerleader and volleyball player. I enjoying taking bike rides, jogging or walking around town, and practicing tumbling in my backyard. I try to drink as much water as possible, but i do tend to drink diet coke too. I eat a LOT of fruit, mainly strawberries and pineapple. During the winter and fall seasons i always crave cereal, but in the summer i only enjoy lighter foods. This started when i was 13, and i am 14 now. I have a strong build, and usually tend to run underweight in the summer but a little "fluffier" in the winter. I weigh about 122 lbs., and i've lost about 13lbs since last november, when i gained my freshman fifteen. I'm not complaining, but can anyone explain this to me so i can understand it and keep it going all year round? Thanks!