I used to masturbate and now that I have stopped, my testicles sometimes become bigger in size and also pains me a lot. Please, what causes this?
I suspect that not masturbating and having testicle pain and swelling are not related. Men have to ejaculate periodically because the seminal vesicles, which are inside your groin get too full. When that happens you will end up ejaculating one way or another.
Swelling and pain, however, are signs of an infection. You should see a doctor about that and don't put it off.