Yes, the sores can be from the Azathioprine. My doctor prescribed a mouthwash that patients on other forms of chemo therapy use. It is a mixture of Nystatin, Benadryl and lidocaine. It works well and now that I have been on the Azathioprine for a while the sores have gone away. I also had the sores on my skin that you are describing - kind of like a cross between boils and hives - they have gone away now too...but I did go to the doctor several times before we got it under control.
Gabe