Something you came into contact with?
Let's start with the basics: Did you get a rash by coming into contact with an irritant?
Think about any new product you're using that contains a fragrance, for instance. Or it could be a new cosmetic you're trying out.
Some people are allergic to rubber ingredients, which can cause an immediate itching or swelling.
There's also irritant contact dermatitis, less severe than the allergic kind. People can develop rashes from prolonged exposure to mild detergents or solvents [source: Cleveland Clinic].