I would strongly recommend that you go see your primary care doctor
about this issue. Any persistent or chronic pain in the testicles needs to be checked out to make sure there is no serious medical problem that needs attention.
The most likely cause of the type of pain that you are having, that is worse with activity but better when at rest, is a hernia
. A hernia is a condition in which a small defect in the muscles of the abdominal wall allows abdominal contents, such as intestine, to bulge downward towards the scrotum. The can often be seen as a noticeable bulge inside or just above the scrotum. However, in very early cases, the only symptom may be pain with exercise.
Your doctor can detect a hernia by examining your testicles and scrotum closely and by observing for a bulge when you strain or bear down. If a hernia is detected, then you will need a surgery
to repair it. Most of the time, the surgery for a hernia is very straight forward, and it doesn't even usually require that you stay in the hospital overnight.
Start by making an appointment with your doctor as soon as you can!