Radiating pain in left testicle.what could it be?

Many things: From a simple muscle pulled to a serious hernia issue to a tumor..... Especially if associated with a bulge should prompt evaluation.

Severe Pain?: 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