Why is there some sand-like substance in my bed after sex?

my girlfriend and I often have sex, and maybe one out of every four or five times there is a somewhat sandy-feeling substance in the bed afterward. I am assuming it's basically some type of vaginal discharge that has dried up.

it's not a huge nuisance or anything, though she feels a little self-conscious about it. I'm curious if anybody else has experienced this? and why it only happens sometimes.