Hello!
To start off with, you should go see your doctor to be evaluated. There are a variety of things this could be including:
1) torsion of the testicle. This would be a surgical emergency because your testicle could lose blood supply and die
2) epididymitis. An infection of the skin surrounding your testicle. Easily treated with antibiotics
3) Hydrocele. Fluid collecting around your testicle, usually managed by a urologist.
4) Inguinal hernia. This is an outpouching of your periotoneum into your scrotal sac.
Regardless of what it is, you need to make sure it's NOT torsion, because this would be very dangerous.
Good luck.
Doctor (MD)
Specialize in internal medicine and pediatrics, also have a PhD in pharmacology