It is difficult to say what has caused it. From your description it does sound like a mark that has been left behind from contact with something that caused the irritation. I have seen this happen with someone that put a small suction cup on the forehead as a joke, and the mark lasted for over a week before it disappeared. You can put some ice on it for very short periods at a time. You can cover it with some concealer or makeup. If it is still present in another week then you can get an evaluation from a dermatologist.