What could cause testicle pain?

Testicular pain: Severe testicular pain can be caused by torsion, if pain of sudden onset. Also torsion of appendix testis. Epidiymitis due to inflammation causes severe pain. Epididymal cyst or spermatocele can be tender & painful. Hemorrhage into testis tumor can be painful. Hydroceles & varicoceles can cause mild pain. Engorgement of testicular veins so called "lovers nuts" or "blue balls" can be very painful.

Lots of stuff: There are many things that could cause this but you should be checked for an infection and to make sure that the testicle is not torsed which can be a surgical emergency, http://www.healthline.com/health/testicular-torsion#Overview1