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Hello and welcome. I'm Dr Chip
Is the pain actually inside the testicle or is there tenderness behind it?
This is likely epididymitis--an infection in the testicle that can be treated with a course of antibiotics
It also could be a prostate infection with pain referred to the testicle and that would need antibiotics, too.
You need to see a doctor for an exam and urinalysis to get this treated.
Please let me know if you need any more information