When did you start the weight lifting? Do your testicles seem to hurt more after you've been lifting?
Is there anything you can do to relieve the pain? Does it feel better standing or lying down?
Are you sexually active? Does it occur after sex, including masturbation? Do your testicles feel "swollen" when you have the pain? Could it be "blue balls" (vasocongestion)? Do they feel better if you ejaculate?
A great many males have some pain during adolescence in their testicles. Fortunately it usually resolves itself.