I wish I had an answer for you. It sounds like you've been on the antihistamine 3-4 days now. If you haven't seen any relief, you may want to call your doctor back. They can sometimes give you a pill that is for itch/anxiety that will help the itch until the antihistamine kicks in. I know it can drive you crazy. It could be this is not the right antihistamine for you.
Good luck!
Carla