its most likely either the result of her increasing thirst diminishing her unusual resistance to a classic vampire weakness or her being in denial do to her sudden case of amnesia gave her limited immunity but the moment she starts doubts that she's human then the weakness flairs up... that could be where her resistance or cope is the strongest?
The shadows are definitely drawn wrong in the two panels after the one where she's running and thinking "I'm a human!"
In the panel where she is questioning why she is getting burned, her own shadow is isolated from the shadow she ran from, which suggests that all her exposed skin should be burning, like you said.
That said, it's a pretty tiny error, and I felt it was fairly obvious what they were aiming for.