It's a common problem when stopping breastfeeding,  especially all of a sudden. If there is fever, there is every chance of an infection. I would suggest not to express out milk, wear tight enough breast support and start an antibiotic like augmentin. If not better with these measurements in 48hours, you need an ultrasound of breast and surgeon consultation and may be blood tests.

It may be breast engorgement, abscess express breast milk, support your breast, do fomentation,take antibiotics, painkillers as advised by your gyanaecologist,stop breast feeding gradually not immediately

Looks like a breast abscess or mastitis or over engorged milk in breast