Because she had a history of under-eating on her vegan diet:

"Anne went vegan during training for The Dark Knight Rises"
http://perezhilton.com/teddyhilton/2012-12-03-anne-hathaway-vegan-diet#ixzz3JHKBP4hs

Dark Knight was released in July 2012.


Anne Hathaway On Starving For 'Les Mis': 'I Just Had To Stop Eating'
http://www.huffingtonpost.com/2012/12/07/anne-hathaway-starved-les-mis-stop-eating-for-total-13-days-shooting_n_2257258.html

Les Mis was released in December 2012.


It's epidemic for actresses to calorie-restrict to maintain being thin for roles and public appearance, crash dieting if they have to, so she most likely was doing what all ex-vegans at this site were doing:

All who got sick when Vegan under-ate
//www.curezone.org/forums/am.asp?i=2162861

When you don't get enough calories, you'll soon feel like this:

“I just didn’t feel good or healthy, not strong,” she said.

It's just like Paleo-queen, Denise Minger, who blamed her vegan diet for getting her sick, but it was clear she simply wasn't getting enough calories on her really strict raw vegan diet:

//www.curezone.org/forums/fm.asp?i=2083438


And don't forget:

Biggest diet culprits: under-eating & low-carb
//www.curezone.org/forums/am.asp?i=2137749