It is not abnormal for babies to develop rashes in their neck area, especially during teething when increased salivation can irritate the skin. You can try using a moisturizing cream to help with the irritation and avoid rubbing the area with a towel or bib, as this can worsen the redness. If you suspect a specific food may be causing the rash, you can try reintroducing it in very small amounts to see if it has an effect.