Signs and symptoms of mastitis can appear suddenly. They may include:

• Breast tenderness or warmth to the touch
• Breast swelling
• Thickening of breast tissue, or a breast lump
• Pain or a burning sensation continuously or while breast-feeding
• Skin redness, often in a wedge-shaped pattern
• Generally feeling ill
• Fever of 101 F (38.3 C) or greater

Milk that is trapped in the breast is the main cause of mastitis. Other causes include:

• A blocked milk duct. If a breast doesn't completely empty at feedings, one of your milk ducts can become clogged. The blockage causes milk to back up, leading to breast infection.
• Bacteria entering your breast. Bacteria from your skin's surface and baby's mouth can enter the milk ducts through a crack in the skin of your nipple or through a milk duct opening. Stagnant milk in a breast that isn't emptied provides a breeding ground for the bacteria.

Lymph nodes and lymph ducts

The lymphatic system is a network of lymph nodes and lymph ducts that helps fight infection. Lymph nodes — found under the armpit, above the collarbone, behind the breastbone and in other parts of the body — trap harmful substances that might be in the lymphatic system and safely drain them from the body.

Risk factors for mastitis include:

• Previous bout of mastitis while breast-feeding
• Sore or cracked nipples — although mastitis can develop without broken skin
• Wearing a tightfitting bra or putting pressure on your breast when using a seat belt or carrying a heavy bag, which may restrict milk flow
• Improper nursing technique
• Becoming overly tired or stressed
• Poor nutrition
• Smoking

Mastitis that isn't adequately treated or that is due to a blocked duct can cause a collection of pus (abscess) to develop in your breast. An abscess usually requires surgical drainage.

To avoid this complication, talk to your doctor as soon as you develop signs or symptoms of mastitis.