Bottom Line: Not enough phosphorus, which is used in the universal energy molecule, ATP.
Phosphorus is an essential mineral just like calcium, and is used for all kinds of things in your body. Not only will you be low on energy without it, but a lot of other things will go wrong too. The best sources of phosphorus are meat, fish, poultry, and eggs. So, if you give those things up, you might run low on phosphorus and start to feel un-energetic.
The cure is very easy; you can google-up some high-phosphorus vegetarian food choices, order a phosphorus supplement, or simply drink some Dr Pepper. Many sodas contain phosphoric acid, but check the label to make sure because some of them, like Sprite, use citric acid instead.
I googled around before writing this, and as far as I can tell, I am the first person to make this discovery. I am claiming to be the originator of this concept until proven otherwise.
Note: I am no longer a vegetarian. I have since discovered that people with Type B blood such as myself must avoid chicken. I can eat any other kind of meat and feel fine. I learned this from Eat Right For Your Type by Peter J. D’Adamo, a diet book that you should read if you want your body running at 100%.