Could most likely be one of three things. Epididymitis, Prostatitis or a pelvic structure dysfunction. I would zone in on Epididymitis though and see a Dr ASAP! If left untreated, it can cause permanent damage. It is an infection of the tube that runs along the back of the testicle tahe carries and stores sperm. Normally affects males 20 -39something and is most commonly causes by a bacterial infection or an untreated STD. Antibiotics and anti-inflamatories are used to treat. See Mayo Clinic website for good info on this.