You're being so self-centered and cold and insincere right now!
What's with you today?
You're acting completely out of character.
普通によい態度があった彼が、今日だけで悪い態度があるなら、「What's with you today?」や「You're acting so weird today.」と言います。とてもひどくなったら、もっと直接に「You're being so...」と言えます。
「How can you be so cold?」や「I've never known you to be so self-centered.」の言い方も使えます。