When i ejaculate my left testicle hurts in the bottom. what could this be?

Epididymitis: Epididymitis is inflammation of the epididymis, which is the tube from the testicle where the sperm travels through before you ejaculate. It can cause pain with ejaculation, and can cause a swollen and painful scrotum. It also be due to infection, which may be caused by sexually transmitted diseases. Your physician will be able to diagnose you with this via a urine test/ultrasound.