I know there are many symptoms for an allergic reaction. Some people may only get eczema, or diarrhea, or vomiting, while others may have trouble breathing or swelling of the lips, etc. As well the severity of these symptoms may appear different in other people.
The difference between and allergy and a sensitivity (or what we term as Intolerances) is an allergy involves the immune system while intolerances do not. A sensitivity, or Intolerance, will not lead to an allergy. However it may indicate the person may be prone to having allergies. Both can exhibit the same symptoms, but Intolerances cannot be tested through normal allergy testing. However they can be tested by other means. Both are serious conditions, but affect the body in different ways. Allergy may or may not cause instant sever problems, while Intolerances can become sever if left untreated as it can affect the intestines as it does attack the lining. Mucus in diarrhea can definitely be caused by an allergy, or an Intolerance, or as you say swallowing lots of mucus while experiencing a cold.
For me, though, I usually also look for the eczema to start up the next day. Usually that indicates either an allergy or Intolerance, I'm still unsure which one. Oh, and of course he later becomes high congested which will last a couple of days. If he continues to eat even something he is Sensitive/Intolerant to he will always be congested and will have eczema over most of his body, or as I term the suit of armour, which also opens and weeps, really gross.