Skid marks in underwear??

Idk if I'm the only one but hopefully someone can help me out . Im a super clean person but for some reason after a couple of hours I get these brown skid marks / stains in my underwear and they aren't easy to wash out . Please explain what's going on ?! I don't have any weird symptoms or foul smell . But it's embarrassing to have to wear only black underwear around my boyfriend because of this .