Her face is extra-smooth and shiny. So we get to see random light sources reflect on it.
It's because she is very satisfied.
It's the kind of face one is supposed to make after sex, but the stories where such over-the-top expressions are used, don't actually have sex.
She is glistening, polished. Remember that a few chapter before we had the popular girls in the class talking about how girls in love look more pretty? This is a play on that.