Nurse on that side as much as possible without neglecting the other side. You have to get the milk out as much as possible and get any that is "yucky" out. It may be very clotted, stringy, or a different color (even tinted pink). I found hand expressing while in a hot shower or while applying hot moist compresses to be the best thing. It was quite painful but it must come out or it will cause more infection. It needs to be cleaned out every two hours during the day and every four hours at night. It won't hurt baby to drink it, oddly enough.
One natural remedy that I heard about after I weaned my last baby is to place a cabbage leave inside your bra over the hot spot. There is some chemical inside cabbage that helps stop mastitis. Never tried it but I would if I ever got it again.
Watch your temp. If you start with a fever or feel like you are getting the flu, get into the doctor ASAP. It is not something you can fool around with. I ended up with double mastitis (both sides) and on antibiotics and pain meds for 6 weeks - we even had a culture done. The doctor and nurse were both impressed with how willing I was to give them a "sample" for it! If I had gone to the doctor when it started coming on, I could have prevented a LOT of problems.
Hopefully, it is just a plugged duct and you can get it cleaned out before mastitis hits. Update and let us know.