The Secret Diary of a Call Girl star, who had 1-year-old son Winston with husband Laurence Fox last year, said that she could not believe how quickly the pounds fell off because she didn't go on a strict diet.
Piper told The Times: "I think I was quite healthy, I wasn't massively healthy. I just ate loads of protein. It's either that or speed pills, and the thing is you do actually have to sleep at some point."
Talking about the end of her pregnancy, the 27-year-old actress said: "I became completely neurotic and saw what I'd be like without booze and cigarettes. And it ain't pretty!"
Piper added that after Winston was born, it was "the only time that I was still, with baby, without distractions or confusion. It was just the two of us."