Vascectomy blocks your sperms to discharge. Your pain is a result of too much sperm fluid accumulated in the ejaculation ducts (Ductus deferens) against your Epididymis/testicles. Sexual excitement stimulates your testicular function to accelerate production of sperm and testosterone. Thus, the pressure in the ductus deferens is built to act againstyour Epididymis/testicles. Once your testicular function is slowing down (or dead) and the sperm fluid is absorbed by the body, the pain will go and your testosterone production will drop too.