My testicles start hurting after erection without orgasm. Why?

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Testicular pain after erection may be a signal that you are having epidydymo orchitis because the epididymis and testicles are the organs where your sperms are stored and produced respectively. So if the pain comes when there is a history of trying to ejaculate that may suggest inflammation somewhere in these organs was there a previous history of STI (sexually transmitted disease) before this incident, pain from the testes is usually referred to the lower abdomen. We also need to be mindful of testicular torsion.I advise you to do urine MCS (microscopy, culture, and sensitivity), semen analysis MCS then scrotal scan to see the testicles anatomically for any scarification.