What Causes Severe Joint Pain While On Remicade Infusions?

I have crohn s disease, was diagnosed in 2005. I currently get Remicade infusions every 7 weeks with solumedrol and benedryl pre-infusion, and 3 days of prednisone post infusion. also take mercaptopurine daily. I have been experiencing severe joint pain for more than 6 months. I had an RA factor and FANA both come back negative in December. Also had an Answer IFX done to see if I was building antibodies to the Remicade, that was also negative. I have elevated AST and ALT, elevated ESR, low RBC, Low hemoglobin, low hematocrit, low lymphocytes. Pain scale of 0-10, I am daily at a 3 and that s with taking 600mg advil 3x/day and 220mg Aleve twice a day. My GI doc is stumped as to what is causing the pain since the blood tests for lupus and Rheumatoid Arthritis both came back negative, and he s afraid I am going to cause a GI bleed with all the nSAIDs I am taking, but I am allergic to tylenol so they are my only option at this point. Any insight as to what they could be missing? other tests that could help pinpoint the cause of the pain? Without the nSAIDs my pain is daily at a 5-6, but can get as bad as a 7-8 at times. Oh, and the pain is mostly in my shoulders, but can be from my collar bone/shoulderblades all the way to my fingertips, and in my hips. When it s really bad it almost feels like I am being crushed, so it s not just in the joints, it s a combo of joint/bone/muscle.