It does sound like chostrochondritis which is minor inflammation involving the little intercostal muscles and cartilage
between your ribs. It is very common in kids your age. The pain can occur at any time and usually is sharp and hurts worse if you try to take a deep breath in. It is somehing you will grow out of. However if the pain occurs when you exercise and it is crushing in nature or if you ever pass out when exercising then you should definitely go see your doctor. If you are still nervous and your parents have good insurance go see your doctor and get an EKG and echocardiogram
for peace of mind. They will most likely be normal and give you reassurance.