I also think certain things trigger it.... like you schedule your day a certain way, am cardio, eat this, train now, eat that etc... when we mess up and miss a workout, cheat on our diet etc. it sets something off that makes you feel like you're not doing things right... then you start (at least for me) picking out the things that you aren't happy with (extra BF, lagging body part, too small, etc)
One thing to think about too - most of the pictures you see of fitness models or whoever you admire have trained heavy for a long time, and spent months dieting down to look the way they do in that comp or photo shoot. I'm sure they even have their off days
