Zithromax is one of the few antibiotics I can take, allergic to all the popular ones.
Have you contacted the prescribing physician? If the hives are not going away with Benadryl, they can prescribe you a steroid that should make them go away within a few hours, you may be able to take it now even if you were not able to pre-surgery.
Make sure you inform any doctors that you had this allergic reaction whenever you are being prescribed drugs. Reactions tend to be worse with each exposure. If you had an allergic reaction to Zithromax, you should not take any antibiotics in the Erythromyicin (sp?) class of antibiotics.