Junk food
I may have missed something, but is Amy's constant gorging on junk food in the 2nd half of the movie ever explained? I can think of a couple of reasons behind it:
1. She plans to kill herself in 2-3 months, so is simply eating what she wants (junk) as there are no long term health issues to be concerned about.
2. She wants to blend in with the hillbillies, who probably eat similar amounts of garbage (or so she assumes)
3. She wants to increase her overall body mass, thus lending credibility to her original story that she was pregnant when Nick 'killed' her and dropped her at the bottom of the ocean.
#1 seems the most likely, as she seems to stop eating the junk and gets herself back in shape straight after Nick's TV interview, where her plans change and she decides to go back to him after he gains public support.