I'm a peer breastfeeding counselor with Nursing Mothers Counsel (Bay Area), and I, too, would say it could be a plugged duct. Your son preferring that side doesn't eliminate the risk of a plugged duct, but they are usually fairly simple to clear up. It's a matter of unplugging the duct. If possible, try to get your son in a position where his chin puts pressure on the spot. If that's not possible, you can use heat and gentle compression to ease the plug down the duct.
Is the pain shooting? Is it aching? Regardless, if you're still experiencing it in a day or so, it wouldn't hurt to call your doctor to see if you can get in a little sooner. Mastitis will show other signs, such as feeling like you have the flu and will include a fever.
Is your doctor breastfeeding friendly? I ask because breastfeeding isn't included in medical school, so what doctors know, they learn on their own through classes, etc or what they've accrued through their experiences, which could be up to date or not. Hopefully, your doctor is breastfeeding knowledgeable :)