If you're not cleaning her neck/chest enough it could just be irritation from dirt. My daughter had a fat neck and she would always get rashes because she was a messy eater

We took her in and he has no idea what it could be, said to use a&d Oitmen See If It Helps

I would see the doctor and Bring up the new formula. It could be an illergic reaction to that.

I'd make a Dr appointment

No Insect Bite, She Doesn't Have Any Kind Of Rash Or Blisters Anywhere Else On Her Body, She Just Started A New Formula A Week Ago Other Than That Nothing Else

Have a good look on her hands and feet and nappy area, it could be hfm, or a similar virus.

Other than that has she eaten anything new? Or a possible insects bite?

It's Like On The Side Of Her Lips Like Where You Would Get A Cold Soar

If it's on her lips I'd get her checked for hand foot and mouth disease. But just guessing!