Your symptoms are suggestive of epididymis (termed epididymitis) infection. A prostate (prostatitis) infection can be concomitant with the epididymitis. To diagnose them, following investigations are necessary;
1) a semen culture
2) a culture of the fluid expressed from penis after prostate massage.
3) blood counts which may show high counts of WBC and neutrophils / lymphocytes.
4) ultrasound of testes and epididymis.
The culture tests not only confirm the infection, but also pin point the right antibiotic which would eradicate the infection. Doxycycline in your case may not be the sensitive for the causative organisms and thus had not given the resolution of the symptoms.
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.
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