No, no, it's just her muffin-top doing it's disappearing act. That theory would work, and they probably did use a light source at a low angle, however, the shadow from one shoulder is distorted.

interrobang probably has it right. Another piece of evidence: the shadow behind her butt is above her butt indicating a light from below. Look at the shadows under her boobs or her chin - if the only light was from below, these shadows would not exist.

Yeah, they used the muffin-top-shadow light on her.

Huh? This person has apparently not lit a subject with multiple light sources. This is not a PS blunder, but rather, complex and unreal looking shadows created by multiple shadows overlapping. Duh.