Why does my penis hurt after orgasm while masturbating?

Prostate Pain: Ejaculation can incite swelling in the prostate. This can result in pain in the rectum, penis, testes, behind the scrotum, low abdomen back or groin. The swelling can also result in decreased force of the stream, urgency and sensation of incomplete emptying the best way to resolve this is to abstain for several days and see if it goes away. Aleve/ibuprofen or Aspirin and increased water helps.