Red mark appeared out of nowhere?

I was minding my own business when I noticed a small red mark on my child's face, about half a centimeter below her lip. It's oblong in shape, about a centimeter long and about 7 or 8 millimeters wide. It has appeared out of nowhere in the past 20 minutes. It is dark red, and kind of burgundy. She's 12 years old. I did the gl test and it kind of went away, it faded a bit, then stayed faded for a second or two, then came back to full colour. It is not itchy, inflamed, raised or anything, if you shut your eyes and rub your finger along the skin, it feels like it isn't there. What could it be?