If you don't have period undies, YOU SHOULD GET SOME.
Set aside maybe 5- 7 pairs of undies that you don't really care about getting stained or try using dark colours like black so you can't even see the stain.
I love these ones by UniClo because it's made of this microfiber material, It's really quick at drying so if you need to do an emergency wash, you can do that and it will dry super quickly.