If the only thing she has is a red rash around her mouth, I would not leap to an allergy. Are her diapers okay? Any swelling? Vomiting? Those would be signs of a true allergy (IgE mediated). It may be an intolerance though those signs are usually GI in nature. I would be VERY reluctant to do allergy testing at all. They are notoriously INACCURATE and can lead to removing foods unnecessarily.