mmm....I just saw this post. My LO has a milk protein allergy (basically dairy intolerance). I'm no expert but i would have thought if it was dairy issue it would have been noticed if your LO is on a regular formula. Another common allergy is egg intolerance, was their egg in the slice? Has your LO had egg before? Other than this I agree with MadB that it might just be your LO is unwell. When my LO had feeding issues (I was/am breast milk feeding) I had to stop taking egg and milk and then I reintroduced one at a time into my diet. My LO reacted to milk but not to egg.