Okay, I Am A 19 Year Old Male And For

Okay, I am a 19 year old male and for a while no my left testicle has been bigger than the other, I have checked for lumps numerous time a haven't felt any, I am an athlete so I have been check for that stuff and the doctor has seen, but my left testicle is bigger than the other and feels hard. If someone could tell me anything to ease my mind because it doesn't hurt or I don't have any pain.