If the antihistamines worked on one hand (did you finish the medication on it?) but the appeared on the other it could be an allergy. Look at anything you may have eaten that you are not in the habit of having, creams, lotions or even a different brand of rubber gloves. Some drugs can interact causing such conditions but my advice is to make an appointment with your GP and in the meantime take notes on anything you can definitely rule out, this will help the doctor and may influence possible treatment options.
As has been said upthread, you can't rule out Diabetes at this time and the advice I got many years ago may help you too, it was 'do not put any new symptom down to your condition, get it investigated'. Good luck.
Edited to add, this advice was given to me for a different condition but I think it is wise for all long term conditions. I just needed to clarify that point.