Before my Crohn's diagnosis, I was developing a pain in my right knee. It started off just kinda "bugging" me randomly, but never bad, or for very long. Just passing twinges. Over time, it got worse. Often, it was worse when I was seated with my knee bent. This new knee pain was in addition to other achey joints that had developed over the last few years.
Fast forward to a year's worth of Crohn's treatments: prednisone, Humira, azathioprine, etc., and the vast majority of my arthritis type pains (including my knee, which got so bad I was limping ocassionally) are gone. My GI prescribed Cellebrex to help, but now that I've been established on the other drugs for so long, I no longer need the Cellebrex to keep the pain under control.
I wanted to just give you something else to consider given your history with all the wonderful meds that IBD patients must take, especially prednisone. My daughter developed knee pain, swelling, popping, creaking grinding noises and after a negative x-ray, we too were sent to the rheumy thinking it was arthritis. Thank goodness the doc suggested an MRI because that's when we discovered a problem with her bones. Just to rule out a potentially serious problem, I'd suggest an MRI to put your mind at ease that it's not something like avascular necrosis (aka osteonecrosis).
Since my daughter's diagnosis, we were blindsided by the possibility of this risk after taking prednisone and feel that there was insufficient disclosure of this risk. She was only on it for 4 months, but she was on high doses (90mg) for a couple of weeks. As a family, we are devastated by this (she's 21), and after many months of asking why and being angry, we realize it could be much worse and have turned to raising awareness and trying to get the prednisone label to disclose this risk. The label warns of osteoporosis, bone pain and fracture, but not that your bones may die. I try to spread the word so that other's are informed and can be on the lookout for the symptoms and hopefully get treatment early on in the course of the disease progression.