I used to experience pain like that when I was masturbating or sometimes over-masturbating. Sometimes exploding a hard and strong ejaculation could be too much for testicles since thats where the sperm is coming from. Since i stop masturbating, I no longer get those side effects in my testicles.
Check for testicular cancer...go see an MD, just dnt ignore it.
It happens to every man, Don't worry.
Your description doesn't give a lot to go on? In my case I have testicular pain more commonly from prolonged sitting. To avoid pain I take breaks every couple of hours and message the backs of my upper thighs especially to elevate distress.
You didn't mention where on your testicle the pain is? But here's a link I just sent someone else that has pain from their epididmus, you might want to check that out?