I am having an allergic reaction 2 days after having a CT scan done. Is this common and how should I treat it? Itchy red whelps all over me.

Benedryl. Is a good place to start but if you're having shortness of breath, tongue swelling or difficulty swallowing please go to the er or call 911.
Notify your doctor. Notify your doctor as soon as possible about your reaction. You likely have a reaction to the contrast agent used for the ct scan. Antihistamines may alleviate the itching. Some allergic reactions can progress and be quite severe. A life threatening condition called stevens-johnson syndrome can be a consequence of some drug allergies. Allergic reactions are relatively uncommon but may be serious.
Contrast reaction. Make sure to let your doctors know you had this delayed reaction to intravenous contrast. Next time you are scheduled for a ct scan, angiogram, or any xray test requiring intravenous contrast, you should be premedicated with benedryl and Prednisone pills 24 hours prior to the contrast injection to lessen the risk of a more severe allergic reaction (i.e. Wheezing, throat swelling, anaphylaxis).