Being vegan does not make people feel weak, but this is a new way of eating for you, so, for a while, you should keep track of what you eat and compute its nutritional content. Maybe even have one consultation with a dietitian. If you have some preexisting condition, it is one more reason to see some health professional about your situation.
B12 deficiency is unlikely the problem this soon, as has been mentioned, and anyhow, a small amount of B12 will go a long way, and staying in B12 balance should not be difficult.
Rather, I would look at your iron intake, esp. since you've felt less than ok for a while. Iron from vegan foods is less readily absorbed, so you need to combine certain foods. For example, have spinach and season it with the juice of a nice fresh half lemon. Or have legumes and, at the same meal, have something high in C such as fresh tomatoes or an orange or berries.
Do ask yourself why you wish to be vegan, because if your reasons are not completely convincing to yourself, you will be easily influenced by those who believe meat is essential to health.
Best wishes.
Someone mentioned B100:
If it is 100% of the RDA of all of the B vitamins, then it is a good thing, for those whose diet contains insufficient amounts of B vits ( though most vegetarians should be getting enough of all the B's with possible B12 exception).
If "B100" just means 100 mg of each B vitamins, as I have seen in some vitamin stores, then don't buy that. No reason why anyone should need 100 mg in particular, for each of the B's--no rationale at all, other than confusing buyers who think they are getting 100% of all the B they need. Oh, and make sure your B complex contains all of the B's, not just a few.
It is actually difficult to find a B vitamin complex that contains all of the 8 Bs and 100% of the recommended daily allowance of each of them. I found one brand and it is unduly expensive.