Unless you have obvious medical problems, it would be a small muscle spasm made worse by your panic. You are growing. These things happen. Slump on the couch with a book and try to slow your breathing and it will go away.
I sat here thinking what would cause pain on exhaling only and I could only come up with 3 things. In order of likelyness: 1 high blood pressure 2. Pneumonia in that area of the lungs 3. Pulmonary embolism. you really need to talk to your parents asap