It is normal for your breasts to become larger, heavier, and a little tender when they begin making milk. Sometimes, this fullness may turn into engorgement, which is when your breasts feel hard and painful. You also may have breast swelling, tenderness, warmth, redness, throbbing, and flattening of the nipple.
Engorgement sometimes also causes a low-grade fever and can be confused with a breast infection. Engorgement is the result of the milk building up. It usually happens during the third to fifth day after giving birth. But it can happen at any time, especially if you are not feeding your baby or expressing your milk often.
Engorgement can lead to plugged ducts or a breast infection, so it is important to try to prevent it before this happens. If treated, engorgement should fix itself.