I have had this problem in the past with my pittie.
She was eating Blue buffalo Salmon and then Duck. Occasionally she would get them and we realized that there was chicken in her food (We were avoiding it because we saw in many cases chicken and pitties do not get along) We then realized that when we would give her a mostly chicken based treat on top of the food with chicken she would break out.
We switched to all things minus chicken and she is doing very well, no breakouts since.
Your pup may have a different problem but I thought I’d offer our experience ;]
