First off u need to talk to your parents about getting into a doctor and getting and ultrasound. This feeling can be from your testicle turning and cutting off your blood flow; it could also be that u have a infection. What i really think it is, is since u are a an athlete u need to wear support and do not wear boxers.
P.S. I went through the same thing and i told my parents about it and i went to the doctor and the doctor made me go to get an ultrasound and it was fine. So the doctor knows i do track and thai boxing and he told me to wear more support. Don't keep this to your self tell your parents and get to the doctor.