I think any diet change makes you feel like crap. If you have pets you will know that when you introduce new food you should mix it slowly in with their old food to make the adjustment easier on their tummies. I'm not recommending you mix in your old food tho.. just giving that as an example.
I felt tired and weak for about 3 weeks when I first switched to vegan. Since then it has been all good. Make sure you are getting your b12, eat lots of green leafy veggies and legumes for iron, and make sure you are getting enough iodine.
It will get better. Hang in there.