With no redness or fever or cold/flu symptoms I'd think maybe a blocked duct or engorgement rather than mastitis. When I finished one of mine went quite hard and sore, I massaged and expressed in a warm shower and the blockage came out (it looked like curdled milk, yuk) and I had no problems after that. Maybe try that?
Righto, express and massage here we come! Feel a bit sad as stopped bfeeding as supply was diminishing and with two other DC I just couldn't dedicate sufficient time to replenish supply etc. Ho hum, at least I got a month in.