Brief Answer:
Concerned for testicular cancer

Detailed Answer:
Hello, I'm Dr. Branch, thanks for using Ask a Doctor. There could be many causes of acute scrotal pain, but especially if there is a definite growth or mass, I would be most concerned about testicular cancer, which can sometimes cause acute pain as well. Otherwise, acute epididymitis is a more common cause of acute scrotal pain, which usually has swelling as well, and is treated with antibiotics. Testicular torsion is also very painful and has swelling, and is treated with surgery.

You are likely going to need to be examined, along with blood tests and possibly a scrotal ultrasound, in order to determine the cause for this. If you would like to give more information about this growth (how long it has been there, has it been gradually growing, are you having fevers, are you having any urinary symptoms) and send a picture, I may be able to comment further on what it is likely to be, but regardless it sounds like you will need to see a doctor in person.

I hope that helps, please let me know if you have any questions about any of that, and I would be glad to discuss it with you further.

---