It just swells?
Is it the testicle it's self or the skin that holds the testicles?
There is sperm being produced there so it may just be that that you feel. If there is any immense pain associated to it I suggest you see your doctor. One testicle is some times bigger than the other, that's normal.
If your really worried about it ask your doctor, you can tell them you masturbate its not really that big of a deal. You can tell him/her not to tell your parents then they cant do anything about that, that's patient confidentiality.
Masturbating doesn't really hurt you in anyway unless it becomes addictive. You are addicted when you begin to neglect things that are important in your life. Like family, friends, and stuff like homework. Masturbating also helps keep your prostate healthy, it shows to reduce the risk of prostate cancer. Like the saying goes, use it or lose it.
Hopefully this helps, If it doesn't just ask try to be more detailed though lol.
