Zithro was the worst for me of all the Abx that I had been so far (I was on Doxycycline for a while, and now I am on Ceftin.) I had a really bad brain fog, imbalance, space out feeling.
It might mean herxing, it might mean that it works better than the other Abx, but how do you know for sure???
The diarhea though might be because you are not taking enough probiotics. It VERY important to take around 3 to 5 billion cells of good bacteria every day, and at least 3 hours away from antibiotics.
Also make sure you are detoxing, since all the toxins that are released because of the Abx might really overwhelm your liver. Get some liver cleansing supplement from some health food and supplement store or order it online. They usually include Milk Thistle. And drink A LOT of water and tea with lemon.