Well, there's also the psychosomatic part of it. You're starting something new, whence you're feeling a little upbeat, and you think you're experiencing a boost in energy levels.
On the other hand, it is possible that some components of meat, if absorbed in excess, are poisonous for the body (e.g iron for the heart).
The whole point in absorbing animal proteins is that they are assimilated by the organism much faster than the vegetal ones; in this way, I am rather skeptic regarding the so-called benefits of veganism. Also, keep in mind one essential point: you feel physically better doesn't mean your health is getting better. For instance, when you take drugs, you feel great. But what's the impact on the long run ? Is that wise to follow a path which has not been correctly investigated by the community yet ?