I already tape my thumb before practice and games, but that does not always help. Any time I hear of an injury that occurs during the adolescent growth period, I always suspect growth plate damage. However, not all damage occurs in growth plates. The thumb is a unique digit. It does not have a middle phalange and it does have a sesamoid bone.
Sesamoid bones develop in tendons.
They protect tendon from excessive wear while they rub against the edges of two bones across joints. By changing the angle at which tendons cross joints, sesamoid bones also increase leverage. In males, the thumb's sesamoid bone starts to calcify at about age thirteen. Before that, it is cartilage lying in the tendon that flexes the thumb. It is possible that you injured this sesamoid bone while it was cartilage such that it developed incorrectly and irritates the tendon in which it lies.