Why do i get pain in my upper part of my testacles after masturbating?

Muscle contraction: The epididymis is a tightly coiled tube that is connected to the top portion of the testicle which uncoils to become the vas deferens. This is where sperm enter from the testicle and mature before mixing with fluid from the prostate to become semen. When you ejaculate, the epididymis, vas, prostate, and urethra all contract to expel the semen. You can experience this as an achy pain afterwards.