The 38-year-old played an alcoholic country singer in the new movie, and was asked not to hit the gym during its making.
She told Contact Music: 'At first I was a bit panicked about it, so I'd start working out a little bit. A lot of people lie the other way and say, 'Yeah, I work out five times a week,' when they actually don't, whereas I was saying, 'No, no. I didn't work out,' when actually I was on the treadmill. It was then that I was like, 'Right, I have got to stop.' But it wasn't until we were finishing the movie that my body completely fell apart. My body was doing OK right up until the end. Then it just started falling to pieces, so now I am left with bulging problems.'
Asked how she manages to stay slim, she added: 'Exercise! Painful, horrible, long exercise. I know from experience that there are no free rides in this world.'