A couple of useful points in a BMJ article:
Mastitis vs. inflammatory breast cancer
Mastitis is more localised, while inflammatory breast cancer can cause redness or skin changes of the whole breast. Inflammatory breast cancer can also cause a general increase in size of the breast, which mastitis doesn’t. Inflammatory breast cancer is not typically associated with fever or leucocytosis.
Treatment of mastitis
Co-amoxiclav. For a penicillin-allergic patient, try clarithromycin/erythromycin + metronidazole. If not responding within 48 hours, consider inflammatory breast cancer as the diagnosis.