I am often asked what it means to "herx" or to be herxing. Here are some explanations I found online. Basically it is a worsening of symptoms due to the antibiotics doing their job & killing off the bacteria. The die off of the bacteria causes your body to become "toxic". The kidneys & liver can only do so much to detox you, so I must drink a lot of water, & take hot baths to make myself sweat. If the herx makes me feel nauseous, I take activated charcoal. That seems to help.
It's hard to tell the difference between regular lyme sypmtoms & a herx. So herxing is good. If indeed that is what it is. But your doc does not want you to herx "hard" because it further weakens the immune system.