A sudden widespread rash can be caused by many things, from allergic reactions to irritants, infections, or even heat. It's your body's way of reacting to something. When a doctor assesses a rash, they will typically ask about its onset, how it's spreading, if it's itchy or painful, and if you've had any recent exposures to new foods, medications, plants, or products. They'll also consider if you have any other symptoms like fever or swelling. It can be helpful to note down when the rash started, where it first appeared, and any potential triggers. Seek urgent care if the rash spreads very rapidly, blisters, causes difficulty breathing or swallowing, or is accompanied by a high fever or severe pain.