I get very quick results when I stop foods that have been unhealthy for me and add in healthy foods. But it was mixed results when I first stopped gluten. I went through a week of withdrawal where I was feeling better in a way, but low in energy, craving the old foods that gave me a high. After the first week I felt significantly better with less fuzzy thinking and more energy.
I would generally think it would take longer than 2 days, of eating better, to feel drastically better, because it take time to get toxins out of your body, but maybe you're one of the lucky ones......
Proper food can make your brain work better, can help your body produce more of the "feel good" chemicals, etc. If you feel better, you feel better, so it doesn't really matter if it's in your head or not. Enjoy it and keep eating properly; you should get better and better as time goes on.