I was doing laundry, and my girlfriend's panties were inside out and had a large, slightly yellowish, crusty stain on the crotch. We have not been having sex lately. I know that girls often get stains there from mucus and what not, but because this was crusty. I am alarmed that it might be semen backflow from another man. Are girls' mucus stains ever crusty? If there is semen there, should it smell like it even though it is dried? Please tell me anything you know.