Epididymitis can be caused by other things than STDs. You could have a urinary tract infection, structural problems, a prostate infection/inflammation, or have TB.
When you had the encounter, did you have unprotected oral? That can cause an infection called NGU, which is non-gonoccocal urethritis, meaning any infection in the urethra that isn't gonorrhea. It can be caused by normal mouth bacteria, different viruses usually causing upper respiratory infections, even the bacteria that causes strep throat. Did they do any other testing on your urine?
If you didn't have unprotected oral, you can't have gonorrhea, chlamydia or NGU from this encounter.