That could be multiple things. Fasting in general will improve mood. I noticed the same thing and began intermittent fasting. Felt great until I ate. Then I learned part of the reason was because I was switching into keto mode while fasting, which also improves mood. So then I switched to the keto diet to try and capture that feeling full time and not just when I'm fasting. In general I feel a lot better. But even though I do keto and intermittent, eating very little processed foods, I still feel best while fasting. There's a lot going on but I think everyone should experiment with fasting. It isn't for everyone definitely, but some people naturally feel better managing their food intake in that way.