It seems to be the same thing that happens to me as well. I was already vomiting a lot before I started treatment. Now on Doxy and Plaqunil, I have experienced some intense herxing. Vomiting seems to be part of it. I wonder if it's all the toxins getting overloaded in our systems?
It just comes out of nowhere too, or I can feel it building for a while. Sometimes I feel better afterwards, sometimes worse. What I do know is that Lyme has made me get over the fear of vomiting!! I am about to add Ceftin into the mix and we'll see what that does.
Keep detoxing as much as your able, I am finding this is very important.