When I had a plugged duct I would use heat before feeding then line my baby's jaw pointing at the area.
This can work really well, especially if you 'dangle' to use gravity. So lie him on the bed, for example, and then feed over the top of him, dangling the breast so his jaw points at the sore part. Not terribly comfortable, but gravity is great
Also the warm before and cold afterwards and only hand express to comfortable rather than pumping to empty. If it's sore and you're not contraindicated (asthmatic, stomach ulcers etc) you can take some ibuprofen.
Keep an eye and if you start with a fever/shivers, get yourself to the GP.