Since it's based on sulfa, you might be having some kind of allergic reaction to it.

Sulfasalazine is an antiinflammatory which are extremely hard on your stomach. I take an antiacid medication a half hour before taking medication it helps. zantac, ask you General practitioner.

Any right upper abdominal pain. I have had nausea for 2weeks, no taste buds.then developed abdominal pain . The doctor said "this will pass" I went home and ate a hamburger. I wanted to die, pain in my abdomen radiating to my back, like a bowling ball stuck under my rib cage. Look up auto- immune pancreatitis. It could be just the medication, but just wanted you to be informed.

Sulfasalazine is a no no for me due to allergies. I ran into the same problems you're experiencing with methotrexate and prednisone. My "perfect cocktail" is 8 methotrexate and 3 prednisone. Heaven for a week. Backed off the prednisone and everything is back to before. Even with the methotrexate staying the same. As far as eating goes many a time I go without a meal due to not feeling up to eating, not tasting good. but later I would grab a sandwich to cushion the blow of the meds and it usually settles down