This happens to me all the time. It even happens with different mirrors. I'll think I look ok at home and see myself in a mirror while out and am like, "WTF??"
“Life is not orderly. No matter how we try to make it so, right in the middle of it lose a leg, fall in love, drop a jar of applesauce.” - Natalie Goldberg
For me, when I am heavier than I want to be, I can see it equally well in pictures or in the mirror. When I am losing weight I can only see it in pictures.
I always look like a damn cow in pictures. Always. And then I go look in the mirror and don't see the four chins I had in the pictures, and I swear my thighs don't look nearly as gigantic as they do in pictures. I've never understood this!