The catalyst was a fight between the 18-year-old Disney Channel star and a dancer on the Jonas Brothers world tour, but the problem started the night before, a source close to the singer's family tells PEOPLE.
"As she did on most nights off, Demi invited her band, dancers and her parents to dinner," says the source. "Afterward, she broke off with a very small group."
The following day, tour managers and Lovato's stepfather, Eddie De La Garza, questioned the group's conduct. According to a source close to the situation, the group had been partying.
"When tour management found out about this, they talked to the people involved, including Demi," says the source. "Demi reacted badly and perceived that someone on tour had told on her."
When Lovato and other tour members were on an airplane that same day, Lovato confronted a young dancer, whom she thought might have been the one to blame for telling on her. "There was a short, physical altercation," says the source, adding that it was "one-sided."
"Afterwards, Demi felt awful about her behavior and realized she needed to take personal responsibility for it," says the family source. "Because of this, and because of the battles Demi has fought throughout her life, both she and her family realized she needed to take steps to get help immediately."
The singer-actress "decided to seek medical care for her physical and emotional issues at a treatment center," says the source, adding, "she is not in drug rehab."
Contrary to published reports, Twilight star Ashley Greene, who is dating Lovato's ex-boyfriend Joe Jonas, "played no role in this," says a source close to the situation. Nor did the Jonas Brothers. "They were not involved in the incident," says another source.
Adds a rep for Greene, "There was never an issue between Ashley and Demi, and there is no truth to any rumors about a conflict."
With reporting by ANNE MARIE CRUZ