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Well, a congenital hemangioma can surface a few days/months after birth. However, a sudden blister could either be due to injury while breast feeding, allergy to some cream/lotion/oil or an infection.
Yes, it is important to show this to your child's pediatrician. Please do not ***** it/ scrape it or rub it. If it is infective it will spread, and if it’s a hemangioma it will bleed.