First of all, don't eat fast food.

Now that I have that off my chest, I think waht you're experiencing is the good old spike and crash. Fast food and other foods made up primarily of high-glycemic carbohydrates [simple sugars, complex carbohydrates without fibre, etc.] pump glucose into your bloodstream very quickly, creating a spike and giving you energy. The problem is that, what enters the bloodstream quickly passes just as quickly, leaving you with a glucose deficit, if not hypoglycemia, that will certainly be noticeable as a drop in energy particularly in relation to the prior quick surge.

So... don't eat high glycemic foods alone...

And don't eat junk food.