What Causes Brown Patches On Skin Due To Tight Clothes?

Hello, I've been wearing several bras that have been rubbing in two certain points for the past couple weeks. Recently I've found that instead of just the usual red marks you can get from a tight bra, there's two patches of brown skin that look like birth marks. They're only small but still causing me to wonder what they are. And no they're not rub of colour from the bras or cuts/bruises. Thanks.