It's quite normal - for Lyme. A herx is described as symptoms that increase in severity, or new Lyme symptoms that appear when treating Lyme disease. This is because the Lyme bacteria releases toxins as it dies, and those toxins drive the symptoms to increase.
What are you doing for detoxing? We have some suggestions in the New to Lyme thread, and there is this site that has good suggestions as well:
www.tiredoflyme.com/detox-methods You will want to add in more detoxing routines, even if you are detoxing now, to help combat this increase in symptom severity, although you will want to be sure to only add in one every 3 - 5 days (depending on how your body responds).
Most of us here use many methods to try to get as many detox pathways (kidneys, liver, intestines, bladder, skin) in the body working as well as possible so that we can stay 'ahead' of the toxins and not herx so much.