What kind of person does this to someone else?

My ex left me for someone she met online. Someone she had never even met up until about 2 months ago (this online friend lived 3 states away and my ex lived with me). This is someone she lied to me about repeatedly and denied ever even knowing. I had to break her down with proof for her to tell me the truth. Now, they've only been "together" for about 2 months but they were engaged within weeks after flying out to be with her - or at least that what they are telling people. I do not understand her logic...I know I wasn't perfect but I never lied, cheated or betrayed her in any way. Matter of fact all our problems and arguments prior to this were just personality conflicts. I'm trying to put myself in her shoes...how can you go from being with someone you said you loved, laid next to every night, shared a life with, claimed you wanted forever with and then leave this person for an internet screen name, profess to love them and now in love with them enough to want to marry them?