Sorry for the cryptic title, but the true nature of my issue might set off privacy filters...

When I bike for any duration, I develop a dull, aching pain in my left testicle. Standing, rearranging things both help a little, but generally only for a few pedal strokes. I'm tempted to guess it's a twisted testicle but I'm not sure. Anyone have any suggestions?

NOTE: early in the season (or in my training season -- this year *my* season didn't start until a few weeks ago), this pain crops up after 30-40 miles. Later in the season it develops after more mileage--say as high as 100+. I'm somewhat overweight (20 lbs), and I spread fat across my body. I'm inclined to believe it's the rubbing between an overly-large thigh and my, uh, anatomy that might be at play. But I'm totally clutching at straws here.