Some babies get so worked up in protest crying that they vomit. Know that this is not uncommon, especially in babies with a sensitive gag reflex, although we know how hard it is on parents when it happens to their baby! If this happens, try not to rush to her hysterically. Although she’s clearly upset (because she’s protesting change and because she hasn’t yet learned what she needs to do to soothe herself to sleep), she's not sick, and her vomiting does not mean she’s not ready or able to learn. By waiting a few moments before going to her, you avoid reinforcing the idea that vomiting is the way to have you help her to sleep the old way. Pick your baby up, change her, and get her cleaned up; then replace the crib sheet with a fresh one. Try not to engage, which will only make it more likely that she'll cry when she goes back into the crib. Once she's back down, you can resume with your check-ins. (Of course, if you have any reason to suspect illness--if you saw a telltale symptom or two during the day, for instance--stop sleep learning and call your pediatrician.)
What do I do if my baby vomits?