It should come as no surprise that Demi Lovato was jealous and upset when ex-boyfriend Joe Jonas started dating another girl, but X17online now has all the details on Demi's virtual mud slinging contest with Joe's new lady love Ashley Greene, and the starlets are definitely playing dirty!

A source tells X17online exclusively that as soon as Demi found out about the new couple, she sent "a very nasty e-mail" to Ashley, chastising her for "stealing her boyfriend and parading around with him on tour." Like something straight out of Mean Girls, Ashley replied with a picture of her and Joe and asked Demi to leave them alone, which seemingly pushed the Disney darling over the edge.

That wasn't the end of the squabble, as our source tells us that Ashley's presence on the Jonas Brothers' world tour may have caused Demi to drop out as the opening act. The night Demi canceled her scheduled appearance and announced she was entering a treatment facility for "emotional and physical issues," Ashley tweeted a picture of the Jonas brothers from backstage and wrote, "Tonight. Was. Incredible."

Though Ashley's rep says there "was never an issue" between the two girls, Demi wrote an open letter to the Pacer Center's campaign for Teens Against Bullying, saying, "People say sticks and stones may break your bones but names can never hurt you, but that’s not true. Words can hurt. They hurt me. Things were said to me that I still haven’t forgotten.”

Of course, Demi probably shouldn't have gone on the attack and blasted Ashley in the first place, so can you really blame the Twilight hottie for responding?

Whose side are you on -- Team Demi or Team Ashley?