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.