Urticaria: usually a delayed allergic reaction, could be something from weeks ago. Often a reaction to a cold virus, oddly enough.
Contrary to popular belief, most of the serious allergic reactions (rash, facial swelling, anaphylaxis etc but not contact dermatitis) is triggered by re-exposure to an allergen. It's never caused by something new. Dermatitis and eczema can be aggravated by new things however.
Urticaria is down in the less-serious part of the allergic reactions spectrum - benign, but irritating and dramatic. Anti-histamines and calamine lotion can help reduce the itch while you wait for it to go.
Allergies are tricksy.
Contrary to popular belief, most of the serious allergic reactions (rash, facial swelling, anaphylaxis etc but not contact dermatitis) is triggered by re-exposure to an allergen. It's never caused by something new.
Dermatitis and eczema can be aggravated by new things however.
Urticaria is down in the less-serious part of the allergic reactions spectrum - benign, but irritating and dramatic. Anti-histamines and calamine lotion can help reduce the itch while you wait for it to go.