It could be a number of things. Lack of sleep, malnourished through long intense workouts, dehydrated, overtraining.
Ever heard the term hitting the wall? You may be at that point right before it. What this is, is when you push yourself to your breaking point basically. It can be cause by all the things I listed above. I had this happen to me about two weeks ago, in cycling they call it bonking. It's probably the worst feeling ever. You feel extremely weak, fatigue, like you want to vommit, and pass out. I could hardly pedal home. When I finally got home I fell asleep for hours, this was during the middle of the day. I bonked because I was malnourished. I had plenty of sleep, plenty of water, I wasn't really pushing to the point where I felt like my heart was about to explode. It was because I didn't eat during my workout. When cycling long distances, I did about 70 miles...you need to replenish electrolytes. So ya...just thought I would share that. Sure energy drinks help to an extent, but they're usually too dilluted, they're more of a supplment. Because if they contained a lot more electrolytes they'd taste terrible. Try adding salt to your gatorade. Cliff bars, gels, PB&J are all great ways to replenish them on long workouts. I prefer PB&J because it's cheaper.