She became one, yeah. I'm not gonna spill the specifics, cause I know it wants to write it out someday. The short version is that it started with an accident, followed by a period of irresponsible behaviour, ultimately culminating in a critical mass of demonic energy to turn her into a demon (of the lowliest form).
It's actually questionable whether the transformation is over yet, since it's not that long since her wings came in, which was a while after the scales tipped from mortal to imp. Though perhaps that's just a matter of being a demon whose form is in flux, rather than an incomplete transformation? Time may tell.