I especially had this problem as a teenager. Now that I'm an adult I don't have it as much. Like people used to say "smile! why aren't you smiling? what's wrong?" Not now. Now instead I get write ups in my job reviews. "Need to present a more cheerful attitude." Stuff like that...
My husband teases me that I get "zombie face." He will say, "Uh oh, here comes zombie face again." I try to explain to him, it's probably linked to my bipolar, but I don't think he gets it. Also we have a kinect and I play on it and I'll be having a really great time. Then it shows me the video and I'm totally blank faced. I watch it and it's eerie. I'm thinking "uhm... I feel like I was smiling? what happened."
I have lots of pictures from the past of "zombie faced" me. It's weird.