With a plugged duct, there is a hard, tender area (about the size of a pea, sometimes larger) in one breast that may hurt when you press on it; you do not feel sick, just uncomfortable in that specific area. With mastitis on the other hand, a larger part of the breast is painful and tender to touch, will feel warm or even hot, and may look red and swollen. Most mothers will have a fever and feel achy and sick, like when they have the flu. Many mothers will feel sick enough that they will want to be in bed.