For the itching I would recommend making a paste of baking soda and applying that to the area. OR good old Calamine lotion. If you can get your hands on it, Benadryl makes a topical ointment. It helps a lot!
This really sounds like an infection and possibly an alllergic reaction as well to something (welts sound like hives? and spreading down away from the site is more than just contact dermatitis from tape). Have you tried just taking some Benadryl or Zyrtec to help with the itching? My boys get hives sometimes, just randomly, and Zyrtec can really help that constant itch. Also aquafor on top of whatever cream or ointment you use, my son told me when I put that on it really calmed down the itch.
I'm so sorry you are going thru this, but I'm really glad you saw your doc, cause infections from surgery are nothing to mess with!