Did you notice yourself or did someone point it out to you?
Luckily, I noticed it.
There was one other woman in the bathroom, but she was washing her hands. There's about 20 feet between the stalls and the sinks. I caught a breeze in those 20 feet and stopped dead in my tracks to yank the skirt out.
She saw my stealth maneuvers (that's sarcasm, it was painfully obvious), but didn't see my butt.
I have been really, really, really paranoid about my skirt all day now. I keep checking to make sure my ass isn't hanging out.