The reason for testes tenderness may be your young age or recent puberty. Even masturbation for a long time causes testes to be tender and make them weak. Stopping masturbation for a little time will make them normal.
It could be a cyst, epididymitis,std or torsion. If you are having this issue you should immediately seek medical attention or seek advice of a doctor. Some of the above mentioned can cause permenant and severe damage so it should not be ignored, there is only a limited amount of time before this could becomes very tragic.