Photograph your skin changes
In the mobile phone era that should be easily possible.
In many patients, wheals do not occur on a daily basis. So you need to expect that you cannot show your doctor, how your skin condition at the time of an outbreak looks.
When photographing the lesions, take care to make them appear as they really do. Good lighting conditions (oblique daylight, no flash, no neon lights), a sufficient distance (at least 30 cm), and a dark background are helpful here.