Difficult to say without examination.
Folliculitis, an ingrown hair, epididymitis or other scrotal mass are all possible.
I agree with the tests for STDs, and I will also consider a rectal exam to ensure the prostate isn't infection.
Imaging the scrotum is not unreasonable - and this can detect most causes of scrotal lumps - including cysts, cancer, hydroceles, or varicoceles.
You can discuss these options with your personal physician or in conjunction with a urology evaluation.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
kevinmd_b