If your breasts are red, swollen and painful during breastfeeding, you have acute mastitis, which must be treated in time, because mastitis will soon develop into a high fever if it continues to develop, or in some cases, the breasts will become septic, which makes it more troublesome to treat. You can apply warm compresses when you first notice the redness and swelling of the mammary glands, apply antibiotics for treatment, and suck out the milk from the affected breast with a breast pump to avoid the accumulation of milk to aggravate the condition. When this symptom occurs, the affected side of the breast can not let the child eat milk, because the milk contains a large number of inflammatory cells and bacteria, which may cause diarrhea when the child eats it.