Welcome to the forum.
I'm no doctor, but a caring patient who had to figure out my allergy/sensitivity.It My allergist had me keep a Food Journal (see many posts in the search box at the top of the page, aka archives). So start keeping one. Any old notebook or calendar with daily pages will do.
What other ingredients were in the porridge? Was there any possibility of another food, drink or medicine? I have had a reaction to Albuterol after 8 hours, and other report reactions (headaches mostly) as long as 2 days after consuming sulfites.
Ask the manufacturer what ingredients were in the packet, and don't eat another one until you rule out all the ingredients!
Allergists and other doctors are really great, but they can't figure out all the problems in a single visit. We have to do our sleuthing too.
Best wishes. I hope you will let us know how it goes.