It looks like my daughters eczema. It could be certain lotions, detergents, perfumes, etc. flaring it up.

Check with your doctor of course! If its eczema I suggest the aveeno baby hydrocortisone cream. It's worked wonders for my daughter.. or the aquaphor baby ointment! It's awesome!

Looks like a heat rash to me.. but since it is past the nipple line I would see a doctor to make sure.

Bring her in to make sure but it definitely looks like a reaction to something whether it's detergent lotion or bath wash