Hello,
Since the rash on breast is spreading it can be due to dermatitis skin or
cellulitis skin.
Cellulitis is an acute
infection of skin and soft tissues characterized by localized pain, swelling, tenderness, erythema, and warmth.It usually follows a break in the skin, such as a fissure, cut, laceration, insect bite, or puncture wound.
I suggest you an immediate dermatologist’s evaluation and get the skin biopsied to confirm the diagnosis.