There's a few questions about already on this, and lots of tedious details in the science we could go into, but I'll settle for this. Yes, it's a response to falling of track. It's physiological as well as psychological and it's very common. The best advice really is to recognise when your judgement is compromised and not over-react. If you're aware that you've eaten things you shouldn't then just try to give yourself a few days of clean eating and don't listen to those depressive whispers in your head. It's something that gets easier the more often you come out of it.

For some it helps to rationalise the changes. The weight gain will likely just be water retention from eating more carbs. After 6 weeks your body should be more than capable of adjusting to handle a day or two of abuse - gaining fat can really be as much of an effort as losing it. It's natural to question progress, and after eating certain foods the glass always seems more empty than full. You'll feel differently about things tomorrow and the next day, so don't dwell on them.

Again.... You need to burn more calories than you eat to lose weight. If weight is an issue, then you do need to reduce your caloric intake, or exercise more to reach the fine tuned tipping point for your body. If you take in more calories than you are burning, you will gain weight. I think that the Paleo diet offers more hunger satisfaction and energy than anything else out there, but the caloric intake to energy burned ratio still applies.