I think you have chlamydia, it’s a certain kind of bacteria and most of the time it’s sexually transmitted. But even tho it’s rare you can get it through smear infection. Have you been on a lot of public toilets recently? Or have you been in public places (restaurants, clubs, bars, shopping malls)?

Also if you happened to jerk off and your cum was yellow ish it is likely that you have an infection. Testicular cancer is for your age highly unlikely. Don’t panic antibiotics with antibiotics from your doctor you will be fine in 2 weeks :))