The Girls Aloud singer - who took over the role of Princess Fiona last October - is concerned that when her time in London's West End comes to an end in May she will pile on the pounds because she won't be so active.
She explained: ''Being on stage is a great way of staying in shape. We do eight shows a week and my dressing room is up two flights of stairs, so I'm always working out!
''I'm a bit worried about eating the same amount of food when I'm no longer doing the show. It's been such a treat not worrying about what I eat!''
However, weight gain isn't the 30-year-old beauty's only worry, as she admitted her last night in the musical will be an ''emotional'' experience.
She added in an interview with Closer magazine: ''I'm worried about how I'll get through my last night - it's going to be so emotional. But it's the right time for me to go. I won't be doing more theatre any time soon. I need a holiday!''