Another thing to bear in mind that I forgot to mention, is that you have been bitten or stung by a very small bug and not have been aware of this.
But with the itchiness on the lips, it is highly likely to be food related. Did you eat any cheese at the time or any fish or prawns? Some people get a reaction to goats cheese, but may be fine with normal cheese for example.
Even though you have had no problems in the past, does not guarantee that you will not develop an allergy to a food or substance later on.
The best way is to keep a diary of everything that you eat and drink as well as touch to try to find the culprit so that it can be avoided.
As well as foods and drinks and unwashed fruit and nuts to keep an eye on, is anything that you have touched or brushed past. These can be allergies to rubber as in wearing rubber gloves and latex, plants, animals, make-up, perfumes, detergents and so on.
Do you recall being touched by any plant? There are quite a few that can cause severe allergies.
Have you applied any new make up or a new perfume, these too can cause allergic reactions.
Make sure that you always wash your hands before eating and after you have handled raw fruit and vegetables (especially chillie, onion, garlic and other strong foods and spices).
The other thing to question is whether the biscuits had any contamination from nuts or were made in a factory that uses nuts, as some people do have an allergy to nuts.
There also may have been an air borne irritant that may have triggered off your reaction, for example, any spraying of chemicals nearby.
I would say it could be due to additives in the food, but oatmeal does not have any. Still, these are serious symptoms, see another allergist if you are able and ask for the RAST test, this is a blood test that test for multiple foods, it is very accurate, I have had them, they are eye-openers.