A probable cause of this pain is what we call blue balls. This happens when you are highly sexually excited for a prolonged period and do not relieve yourself through ejaculation. In your case even though you have ejaculated, the high excitement through porn may be causing the pain. If this were the case, then there is no reason for worry. Stop watching porn. But need not stop masturbation. Another cause is infection in or around testes. For this you have to conult a doctor near you in person.