oh just googled and found this

Skin Symptoms
Many patients demonstrate outward signs of a penicillin allergy. Raised bumps that are often red and itchy begin to appear on the skin. These bumps or hives can vary in size and intensity and sometimes persist for hours or days. Other patients may experience a rash that appears a few hours or even days after taking penicillin. Itching may occur with or without the onset of hives or a rash.