An allergic reaction is immediate and acute. An intolerance reaction is delayed and soft. You don't know if you are reacting to a certain food. If you eat a food that you cannot tolerate, your immune system will respond by releasing lots of adrenaline and your cortisol levels will go up. Nothing very noticable if it is only the one day. However if you continue to eat the offensive food your stomach will start to bloat, you will produce lots of gas (farting) and feel most uncomfortable. You have to learn the signs telling you something is wrong.

I'm a diabetic type 2 and I notice it the next day because my sugar levels are up. (I test every day). When my adrenaline levels are up (the fight or flight syndrome) the sugar processing plant slows down. While fleeing the lion who wants to have me for dinner, i can"t very well call for a time out to deal with a hypoglycaemic episode. So my body stops processing the sugar in my bloodstream to keep it available for the fleeing part (extra instant energy to burn). It is your body dealing with stress.

The 2 most common culprits of intolerance are dairy and grains.

In GAPS they suggest putting a dab of the possibly problematic item on the inside of your wrist just before going to sleep. If you wake up with an angry red spot then you have an allergy/intolerance to that item. My disclaimer is that I simply read this in the GAPS book and have made no effort to try it or validate it.