Some people believe that you can have both IBS and IBD at the same time. Regardless, you need to go back to the doctor and get bloodwork and a colonoscopy, maybe even a capsule endoscopy (camera pill to check the small bowel out). I'm not a doctor but it sounds like if you did/do have IBD that the drug combination you took, Sulfasalazine and Prednisone, put you into remission for a nice long time OR you don't have IBD and have something else going on. Please go back to the doctor and get this sorted out.
Good luck and let us know how it turns out