Rule out tumor. Any testicular lump must be considered to be a tumor until proven otherwise. Advise see a urologist. Hopefully it is just a cyst. An examination plus scrotal ultrasound usually answers the question. Surgical exploration is indicated if any doubt remains after the above. Good luck.
Testicular mass. See md asap, tomorrow. Testicular cancer is the #1 cancer age 15-39. Very high cure rate if caught in time (grows very quick). It could be an infection. Dont waste time.