You were all probably low on iron, among other things.* B12, Calcium, Omega-3's, and a handful of trace elements are also on that list. Most of those can be solved by supplementation, but iron's more tricky. Likely why life evolved carnivorous tendencies in the first place - the iron is safely tucked away in a readily-absorbed heme complex.
Iron's one of those things your body doesn't absorb easily through supplements, and competes with Calcium absorption (So, no taking it in multivitamins or consuming dairy at the same time you take it, otherwise it will inhibit absorption). It will also chelate with tannins (which are in teas, coffee, wine, and chocolate), which also inhibits absorption. And, for more fun, if you over-do it on the iron, it will horribly poison you, so, uh, basically check with a doctor re: supplements. That said, if you're good about balancing out the protein, iron, b12, calcium, and omega-3's (among others), you'll feel a lot less crappy in transitioning over to a plant-based diet.
*In my semi-scientific, zero-medical-training, not-a-doctor opinion
tldr: try iron next time you go vegetarian!