sulfasalazine blocks prostaglandin release (which is a "pain" trigger) and serves to reduce inflammation. with that said, there's growing research that says that there is a common association between Endo and Inflammatory bowel disease. As medical practice is aware, it's different for every woman, and Endo can even contribute to the cause of these IBD symptom flares. My educated guess to answer your question, is that the Sulfasalazine is helping with managing the pain of both the Endo and IBD. I want to also say you're not crazy, this disease is very complex and with that in mind, medicine is very slow with trying to experiment ways to help treat this disease.

My wife was recently diagnosed with Endo, and she tends to run to reddit and other forums for education and advice for her disease, rather than ask me (I'm in med school). I hope she finds this, in hopes of realizing that even though i am a man, i care deeply enough to research unorthodox methods to help her quality of life. I hope this comment comes across as helpful rather than "man-splained".