Testis can be tender for lot of people. This itself doesn’t indicate a problem. To be sure the there is no underlying problem better to consult physically for a physical examination or time being get a scrotal scan and doppler done and review

You did not mention that since how long you are having this pain on walking or running in day routine without touching the testis do you feel the pain or not meanwhile you can use tight underwear or supporter for the testis that will subside the pain to some extent if it odoes not help can you  go for Sonography of the scrotum that will help u  to point out the exact reason but it seems there is nothing serious

Could be infection of testis,? Orchitis Can be treated with medicines mostly Wear a scrotal support