Might have something to do with your glycogen stores. Sometimes it takes a few days for your body to replenish these so make sure you're not doing too much too frequently. Make sure to drink lots of water too. I believe glycogen is stored in the muscle with water molecules attached at a 1:4 ratio, or something like that, so where there's glycogen, there's water. So you need lots of water!
The same thing happens to me though. Some days I feel really full and powerful and then sometimes I feel "deflated." It's kinda funny because I actually use that same word to describe how I feel.
What helps me is resting up and making sure to stay hydrated. You could try increasing your carb intake unless it's already high enough.