Leftover animal foods (meat, fish) are often the main culprit in leftover reactions.  Some people are more sensitive and react to any leftover.  Usually, 4 hours is the rule for a food to become risky, but if refrigerated at a low temperature, there might be a longer window.

Then there's the "bucket."  If your bucket isn't already close to full, you might not react to something.  If it's filled to the brim, one small thing might cause a problem.

I don't know that a protein shake will be the answer for the long term, unless you're just having them when you're in a flare.  Have you tried a low histamine diet or other elimination diet?  Are you on enough H2 antihistamines and mast cell stabilizing meds?