I believe you mean 6mp, which is 6-mercaptopurine. It takes weeks or months to become completely effective, so I doubt that it is already having such a direct impact on you. Did you ahve a colonoscopy recently? Any structural issues inside that could lead to the proboems you report? Constipation is not that uncommon in Crohn's -- what were your bowel habits before?
Final analysis, if this is a persistent problem, consultation with GI re diet and any supplements like fiber would be wise.