Kristen Stewart in sexy black dress: Even the most laid back actress in Hollywood, Kristen Stewart, has to make sure she’s fit and trim for the big screen — and that means staying away from sinful treats since Breaking Dawn starts shooting in less than a month!

“I want a cheeseburger so badly, but, you know, I have to be a vampire in a few weeks,” the 20-year-old actress told the LA Times‘ Amy Kaufman at lunch in Los Angeles, before choosing to munch on a veggie burger. Although she’s toning up for her role as Bella, her character spends much of Breaking Dawn pregnant with her first child — and Kristen said her fake belly is totally uncomfortable.

“I’ve worn it. It’s … crazy. I’ve done fittings,” she said. “It gets immense. It gets so massive at some point that it actually looks inhuman. Like it’s hurting her. There are striations of bruises.”We can’t wait to see Kristen Stewart “pregnant” with Robert Pattinson’s baby — we’re just bummed we have to wait more than a year to see it!