If you lose a lot of weight, it can take between 1 and 3 years before your mind adjusts and stops acting like you're on the brink of starvation and are simply normal. I'd imagine it's probably something similar for just general diet changes.
If you're talking about feeling like that in the first few weeks, my guess is you didn't eat enough calories on your replacement diet. It can be hard to determine how much vegetables to eat when you're used to them being the side dish instead of the main course.
If you're talking about feeling that way after a few months, it might be B12 deficiency. The only place you can get B12 is through dirt, animals that eat/drink dirt, or vitamin supplements and fortified foods.
If you're not eating animals, and you don't want to be exposed to all the nasty diseases that comes from drinking dirty water, then you kind of have to be on the lookout for getting it in other ways. Definitely one of the biggest nutritional drawbacks of being vegetarian or vegan in the modern day, but it's also pretty easily fixed.