I have a stinging pain right under my left testicle? what kind of disease could it be?

Get it checked: Whatever it may or may not be, if it continues to hurt you should definitely get it examined by a physician. He or she may want to investigate further with a testicular ultrasound. No matter the outcome, you should be examining yourself regularly. Feel the whole surface of both testicles, looking especially for hard, "nodular" spots.

Varicocele: This be could be a varicocele or hydrocele. Other possible could be a infection or less likely hernia. Examine yourself to make sure u don't feel a mass. See a md and you may need a ultrasound.