Alright no she didn't. However, in a recent photoshoot for Glamour about "natural beauty", Demi was suspiciously photoshopped to be significantly more thin than what she is right now.
For comparison, here's an untouched photo
Why was she photoshopped? Why are we still allowing this at all in magazines? Discuss.
It is obviously photoshopped but it wasn't like she really had control over that. Most of the time you don't even get to see the mag before it goes out for print.
Her clothes were also obviously chosen to give the illusion of her being thinner.