2 year old daughter, Bactrim Allergy?

My daughter is 2 years old, and when she was 1 she went in for her well child check up, and was given her vaccinations including the Chicken Pox vaccine. While there they also discovered she had a UTI, and was put on Bactrim. A few days after starting it she developed a few pimple like bumps on her back, we called the dr and over the phone he said she was allergic to Bactrim, thus eliminating her use of any Sulfa drugs. We would like to find out if she really is, or if this was caused by the Chicken Pox vaccine, which we've heard can cause bumps as well. How shold we go about doing this?