01/04/2011 05:12 am ET Updated May 25, 2011

Alex Welch: Demi Lovato's Fight Victim (PHOTO, INFO)

Demi Lovato's decision to seek treatment came after she punched a dancer who appeared on "America's Best Dance Crew" ... TMZ has learned.

Sources tell TMZ ... Alex Welch was on the receiving end of Demi's blow. Welch is a backup dancer on the Jonas Brothers Live in Concert Tour. She was also a member of Beat Freaks, which got the runner-up title on season 3 of "America's Best Dance Crew."

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