If your breasts are red, swollen and painful during breastfeeding, you have acute mastitis, which must be treated promptly, because mastitis can develop quickly with high fever, and some people have pus in the breast, which can be troublesome to treat. When the redness and swelling of the breast is first noticed, hot compresses can be applied and antibiotics can be used to treat the condition, and the milk in the affected breast can be sucked out with a breast pump to avoid aggravating the condition. When this symptom occurs, the affected breast should not be allowed to feed the child, because the milk contains a large number of inflammatory cells and bacteria, which may cause diarrhea when the child eats it.