What Causes Swelling In The Testicles?

63 retired baseball coach 6' 230 good health active - right testicle has swollen to twice size of left (3-4 months) has always been larger & hung lower but is beginning to hurt some- mostly a dull pain but sometimes a bit more - some pressure but testicle is fairly large - tennis ball maybe - left side golf ball size. That's about it. Thanks, Clay