First, the part that isn't about skincare: swelling of the lips, tongue, eyelids or throat, difficulty breathing, or a rash spreading rapidly over your body is a medical emergency, not a skincare problem. Stop reading and get help.
Everything below assumes a localised reaction to something you put on your face.
In the first hour
Stop everything. Not just the product you suspect — everything. Cleanser and a bland moisturiser only. The instinct to keep the rest of the routine and remove one item is understandable and it makes the next two weeks harder, because inflamed skin reacts to things it tolerated last week.
Don't try to fix it with products. No soothing serum, no calming mask, no sheet mask, no ice roller. Inflamed skin is more permeable than usual, which means whatever you apply now has a better chance of causing its own problem. This is the single most common way a two-day irritation becomes a three-week one.
Rinse with cool water and apply something bland and thick. Plain petrolatum works. A simple ceramide cream works. Fewer ingredients is better, and "for sensitive skin" on the front of the package is not a substitute for reading the back.
Over the next few days
Keep it to two products. A non-foaming cleanser once a day, and the bland moisturiser as often as it feels useful. Cool water, not hot. No exfoliation, no actives, no makeup over broken skin if you can avoid it.
Over-the-counter hydrocortisone at 1% can help with contact dermatitis and it's reasonable to use it for a few days. It isn't appropriate for extended use on the face, it shouldn't go near the eyes, and it shouldn't be used if there's any chance of infection — spreading warmth, pus, or a fever means see someone instead.
Working out which kind it was
The distinction changes what you do next.
Irritant reactions appear fast, often within hours, stay confined to where the product was applied, and are dose-dependent — less product means less reaction. Most cosmetic reactions are this kind. They usually settle within days of stopping and you may be able to reintroduce the product at lower frequency later.
Allergic reactions are delayed, typically appearing two to four days after exposure. They can spread past the application site, they often itch more than they sting, and they are not dose-dependent. Once you're sensitised, a trace amount is enough. These take longer to settle — two to four weeks is common — and the product is permanently off the list, along with anything else containing the same ingredient.
The delay is the most useful clue. Something that stung on contact is probably irritation. Something that appeared on Thursday from a product used on Monday is more likely allergy.
When to see a doctor
Blistering or weeping. Spreading beyond the application area. Anything involving the eyes. No improvement after two weeks of doing nothing. Or recurrent reactions where you can't identify the culprit — that last one is what dermatological patch testing exists for, and it will tell you the ingredient rather than the product.
Reintroducing
Wait until your skin has been calm for at least a week. Then add one product, and only one, and wait two weeks before adding the next. Write down what you added and when, because you will not remember.
If you were using six products before, this takes three months. That's the cost, and it's cheaper than the alternative of adding everything back at once and starting over.
