How tell if baby had an allergic reaction to food

So how can you tell that baby for sure has a mild reaction to certain foods like my 11 month old I suspect has been having an allergic reaction due to getting a congested after eating g his porridge with strawberries and nuts in it. Now let me mention that I have given him all sorts of nuts on their since he turned 6 months even strawberries but he never had an allergic reaction so I'm quite bemused at the minute as I haven't given him anything new at all. Today I have given him his prescribed antihistamine but doesn't seem like it made a difference as now I'm contemplating whether it is an allergic reaction or is he teething as he is rubbing his ears. No hives or rash at all after eating though apart from congested nose. What shall I do and should I be trying again with the same food? Please suggest 🙏

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