We can often have a disconnect between synptoms and actual UC inflammation. Happened to me before.
We can get joint aches as an extra-intestinal manifestation of UC. Sometimes it's related to the amount of active inflammation we have: aches worse during flares and goes away during remission. Other times the aches are a constant regardless of how well our UC is doing. Sometimes it's a second immune condition like rheumatoid arthritis.
Here's a link from the ccfa about