"Demi walked up to me and she just punched me. It was like the definition of a sucker punch!"
"Of course there was like a shock thing about it, because I wasn't expecting it, I had no idea why," says backup dancer Alex Welch.
Welch tells INSIDE EDITION's Jim Moret the punch happened on a plane while she was on tour with Lovato and the Jonas Brothers.
"It happened so fast, that's why no one could react. It was one of those things were I was sitting there, she walked up, just punched me really hard and walked away as fast as she could," says Welch. "My face, it immediately swelled up we put ice on it the whole ride back."
"Did she say anything before or after hitting you?" asks Moret.
"No I didn't hear anything from her," says Welch.
Dramatic photos show the bruises on Welch's face, allegedly inflicted by Lovato. Welch says Lovato believed she'd been leaking stories about the troubled star, but Welch says that never happened.
She says, "We were friends, we were really good friends. I think that's one of the hard things, is to think that a friend would do that."
The teen star abruptly dropped out of her tour with the Jonas Brothers in November to enter rehab.
It was hard to believe the innocent little girl who used to sing with Barney the dinosaur could ever find herself in any sort of trouble, but it seems the stress of stardom took its toll.
"She is a teenager, you know? She's going through a lot, she's trying to figure out who she is as a person," Welch tells Moret.
Welch plans to file a lawsuit against the teen star. A spokesman for Lovato says they hope to resolve this matter as soon as possible.
Moret asks her, "Do you feel she owes you an apology?"
"I don't want to force her to make an apology to me, because I want it to be real, I want it to be sincere," Welch says.
Variety reports that Disney plans to resume shooting the third season of Lovato's show in January, whether she is done with rehab or not.