When something very cold touches the roof of your mouth, blood vessels there constrict rapidly to preserve heat, then rebound and dilate suddenly once the cold passes. That rapid dilation is detected by a nearby nerve (the trigeminal nerve), which also supplies sensation to the face and forehead — the brain misreads the signal's origin and perceives pain in the forehead instead of the mouth, a phenomenon called "referred pain." It fades within a minute or two as the blood vessels settle back to normal.