It was being real of you to tell me this dress made me look fat. Thank you. I had no idea you thought that and I never would have known if you went on thinking I would take it personal.
Another way of saying are you being serious?, or are you being really serious, used commonly in a question to ask someone if they're being serious about something or someone. Are you being for real? is commonly also used if they're in shock about something or an situation.