mc7

There are a lot of things that could be (like infection, blockage, or nerve disturbance), so your best bet would be to see a doctor. But if you wanted to try to get some sort of guess about your problem from this forum, you'd have to describe where it hurts and the sort of pain, how long it's hurt and anything else related.

The first thing I would ask is if it could it be an infection? Are you sexually active? Do you use protection? (correctly?)

Does it feel like the pain is coming from the scrotum or behind it? Does it feel tender otherwise? My first guess would be prostate inflammation (probably from an infection). There's no chance you could have injured your prostate, is there? Do you have any other potential symptoms of an infection?

Besides prostatitis there are also other genital inflammations (also most likely from infections) that can cause painful ejaculation like urethritis and epididymitis. If there is no chance the infection (IF it is one) could be any sort of STD, I would guess it's yeast. Yeast infections are pretty common and generally much less threatening than bacterial STDs. Cranberry juice can help quite a bit with yeast and other genital infections if they are minor. (It's not going to cure an STD, though.) Avoiding milk and cream helps to overcome infections too.

ONLY if this pain is very minor, I might just hold off on masturbating and see if the pain goes away in a day or two, but if it actually significantly hurts AT ALL or if it gets worse, see a doctor right away. Or if there's any chance it could be a result of an STD, you will want to see a doctor or clinic. Infections can cause permanent damage if they're ignored.