Can a badly performed root canal cause a skin rash?

There are few dental diseases that could cause a sistemic rash like the one you describe, but they are specific diseases that give many more symptoms that are very characteristic and are easily diagnosed. So i highly doubt that your moms rash has something to do with the dental procedure.

You do not provide basic information about the history of the rash. Dermatological diseases require a close up, physical clinical exploration. I suggest to seek a dermatologist consult. Many dermatological diseases look alike to an unexperienced eye. Skin lesions are common causes of headeache to med students. Dentist are over specialized on thier area so they won´t help you with her disease.

So look for a trustworthy dermatologist. Do not waste more time.

Thank you, we have an appointment with a dermatologist soon. Other drs have just given her steroid creams which make no difference and antihistamine which again had no affect. We believed at first the rash to be shingles but it isn't/

That is a common mistake of doctors that have little experience with dermatology (i know some that have almost null experience with it).

Since many diseases are actually cured with steroids they find easy to prescribe them without doing first a correct diagnosis. Steroids can alter the natural history of a disease and mask the real cause.

My first tought was also of shingles but you are correct, even with the few information it does not seems to me like it.

of course!!

There can be a possibility of skin rash while performing root canal. Sometimes due to some medicines and some other treatments done on tooth causes some unusual affect on tooth and also causes various types of infection. So it will be very much better if this problem can be checked with proper doctor.