I've been vegetarian for 5 years and I was recently told that I have a surprisingly high iron level for a woman in her late 30s :) It seems that women are deficient in it for some reason.

I heard a lot about this feeling of "weakness" or not feeling really full or being hungry constantly when not eating meat from my omnivore friends. I think it all boils down to what you were eating when being vegetarian, and how much. One problem with switching from omnivore to plant-based is not eating enough. Meat is highly caloric, while plants are not, so you need to make sure to eat more and very varied.