The likelihood of an emotional or personality disorder certainly seems high with this girl. I would guess that's the culprit behind her 180 degree turnaround, but with that already having been covered by numerous other posters, let me also throw out there that the folks in a newbie play partner's immediate circle are often as a big a factor as the new partner themselves.

How many of us have ever spoken about something pleasurable and exciting to us with another only to have our confidant look at us as if we had tipped off into the deep end? Someone fresh from the emotion of scening could very well enthusiastically share with another, only to have that other condemn the activity, BDSM as a whole, or the person themselves for even participating. The pleasure of vulnerability can warp to shame and anger pretty quickly at times, especially for those still finding their footing. A new sub or bottom can wonder if there is really something wrong with them for having reacted favorably to what a friend just put down, and now they're blaming the Dom/me or top or whatever.

I think that it's also a good idea to talk about that aspect of of scene - the perception and the delicacies of sharing with others.