What's up with panty crust?

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?...