The thing that bothers me is, she really isn't the kind of girl to cheat.
Don't be fooled by this. ALL girls are capable of cheating, trust me. No matter how sweet and innocent they look or act, they have weaknesses just like we do. You seem to have a handful of evidence to be suspicious. I would confront her and if it turns out you're wrong, just say you two have been distant lately and you just wanted to get to the bottom of things.