It could be the electrolytes.

I notice a difference in my rides when I use electrolyte tablets. I used to suffer from all sorts of stomach upset on longer rides ... started taking electrolyte tablets, and they settle my stomach down and allow me to digest my food better which in turn gives me more energy.

I can also drink more water when I take electrolyte tablets which is good for hydration ... and being well hydrated can increase the energy levels too.