Hello - thanks for asking your question.
Without seeing the lesion, I cannot be sure what it is.
Possibilities for a scrotal mass includes cancer, trauma, epididymitis, or a cyst such as hydroceles, spermatoceles or varicocele. The ultrasound is the appropriate test to take.
It would depend on the area of the trauma, but it would be unlikely that it would cause the cyst - it would already have to be pre-existing.
If the scrotal mass is determined to be epididymitis, that can certainly be associated with and STD. Cysts such as varicoceles or hydroceles are not associated with STDs.
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.
Thanks,
Kevin, M.D.