What Causes Dark Patch Of Skin On Cheek?

My 15-year old daughter woke up with a large dark spot that suddenly appeared on her face. It s an oval-shaped spot about 2-inches X 2.5 inches, and appeared on her right cheek just below her cheekbone. At first we thought she had slept on something hard (she didn t), but as the day wore on, it never faded. It s been two weeks, and its turned a little darker, but it s still there. Her skin is smooth (no rash) in that area, there had been no trauma, and the medication she takes for acne control (minocycline, benzycline) has never caused a reaction (she s been on them for over a year). She hasn t eaten anything different, or used different brands of cosmetics or perfumes. The spot just appeared from nowhere, and it seems as if it s going to stay. There are no lumps, bumps, swelling, rash, veining, - nothing. It s very smooth, but just a few shades darker than her normal skin. Should we take her to see a dermatologist?