That's just the skirt - which looks like it might be denim or some similar fairly heavy cloth - pressing it down though. (Into the buttcrack actually if you think about it...) But there's otherwise a fair bit of free air around it, which wouldn't exactly be the case under trousers...
Also, even in the case you describe it'd still be a decent volume of matter stuffed into the garment. At the very least equivalent to a small towel; you wouldn't exactly want to go around with one bundled up in your pants, nevermind now one that presumably has sense of touch to boot. Should still be pretty visible, too.
Plus kinda hard to see why she'd keep it in there to begin with...