Sounds like a vaginal condom, but thats lame. If magical, then a sorcerous artifact that one can wear, so both physical, magical, and throw-away-able
Whore's shell and Yatwer, thats an interesting question. Perhaps used on a small enough scale that she doesn't notice/care?
My biggest question is how did she acquire and/or afford it? The books make it sound like she just went down to the market and bought one... If it is magical, I would think it would be harder to come by?