The main cause of mastitis is milk trapped in the breast. There can be a blocked milk duct. Mastitis sometimes involves an infection. Possible causes are milk trapped in the breast or bacteria going into the breast.

Symptoms include breast pain, swelling and warmth. There might be fever, chills and a feeling of not being well.

Antibiotics most often are needed if there's an infection. Mild pain relievers can help with discomfort.