That dark patch on your cheek that you thought was a healing sunburn could actually be the sign of a common, though little-known skin condition called melasma.

Melasma causes brown or brownish-gray patches to appear on your skin ? most commonly on your face, along the forehead, cheeks, upper lip and chin. It’s caused by an overactivity of the skin’s color-making cells, called melanocytes, according to the American Academy of Dermatology.

The good news? It’s easily treatable and rarely dangerous. Melasma can range from mild to severe and affects more than five million Americans, 90 percent of whom are women. It is also more common among people with darker skin tones, including Latinos and people of African, Asian, Middle Eastern and Mediterranean descent, according to the AAD.

“Many of my patients aren’t aware their discoloration is a common disorder named melasma,” Lauren Ploch, a Georgia-based dermatologist, told HuffPost. “They often attribute it to a previous sunburn or oral contraceptive pills. Many people aren’t aware that it is treatable.”