Why do I havd pain in my left testicle?

I am 15 years old and every once in awhile for about the last 4 years, I will randomly get pain only in my left testicle. It doesn't happen very often and nothing in particular triggers it. When in pain, my testicle usually swells up and I can barely walk. I've been to an urologist and my doctor several times for this. They still do not know what the problem really is. I had an ultrasound done which cleared up that it is not tortion...they said that it almost looked like something was connected to my testicle that was supose to fall off at birth. If anyone has even an idea please give me feed back..