You mean you don't eat after lunch. That may definitely be the culprit. You need calories to keep you going and to replenish your spent energy. Whats worked for me is munching on something every 1.5-2 hours. I have a drawer filled with fig newtons, nuts, oats and hone bars, clif bars, some cookies, a couple of fruits and beef jerkey.

I remember having no energy one day I missed eating snacks because of a big project. That was a horrendous ride until I stopped for a snickers bar.