It sounds like a blocked duct to me.
Warm pack, feed baby, express, hand express, cool pack.
Take some ibuprofen to help with the swelling - and if you start to feel crap all over rather than just sore boob then get to the doctor for antibiotics.
Ultrasound treatment from physios is very good for clearing blocked ducts - but you need to make sure that your physio has experience with duct-zapping.