Demi Lovato is down in Mexico today, and yes she's 17 and yes these pictures show part of her ass. At first it might seem similar to the Miley stuff but there's a huge difference between a girls vagina and her ass. And if you didn't already understand that, I feel really sorry for your girlfriends ass after sex. I bet she was pretty surprised when it dawned on her you didn't know there was a difference.
(source = splash news online)