i have no idea about the dark spots, i think its tough to say. some people just have 'discolorations,' which in my experience are markings of some significance to the spirit of the person.
i once knew a man with a discoloration on his face, and i always thought it was about certain things he wasn't supposed to forget during this lifetime. or, it was a mark of imperfection. but i don't think marks like this are random.
even though you're somewhat sure that she hasn't had them since birth, how long have you seen her? if they're on her butt, they may have been there for many years without her realizing it.
no doubt if you ask andreas, he'll tell you what organ that area of the body relates to.