Why was her body stiff...

Because it was a dummy/doll. It was the WORST reenactment of a dead body I think I have ever seen. Sure, when people die, they get stiff from rigor mortis. That generally lasts about 72 hours and they become limp or moveable again.
It looked like a doll. It did not resemble a human in any way shape or form and I cannot believe they actually tried to pass that off on camera. It was so fake looking it just cheapened the movie. They couldn't afford something more lifelike??

OK, fine, but that doesn't excuse the cheapness that was done in the film with that horrid doll. I kept laughing and thought, "These people are mourning a doll." I understand potential budget constraints, but come on, there had to have been SOMETHING they could have done in the art/special effects department to make it a little more realistic!