What happened in the space of a week that caused my ex to have a seemingly split personality?

DTMF... That is not the kind of friendship anyone needs to waste anytime on. How he treated you the last time was unforgivable regardless of the reason. The reason doesn't matter. But if you need a reason, more than likely, he is either A) feeling guilty and trying to loyal to current g/f B) He isn't that into her and regrets the mistake he made by getting back with her. C) Seeing you reminds him what he is missing out on, and puts him in a bad mood. D) You are actually seeing his true colors and better off without him. If he calls, are you going to take his call? Why don't you wait and let it roll over to your v/m. If he doesn't call, no worries, you're better off. If you do decide to return the call, wait at least a week and explain that you didn't actually enjoy yourself the last time you two got together. You are curious if what he was upset about or why he was in a bad mood. Don't ask him if he was upset with you. You certainly didn't do anything!