Thank you for writing and congratulations on the weight loss, that's a great achievement.
A very small amount of mucus in the stool is normal, as mucus forms naturally in the colon. But when distinctly seen by the naked eye, it may be due to an infection or other conditon in the digestive system. Since you have
colitis, this may be the cause.
However, there could be a different reason for this - talk to your doctor. Your arthritis could also be a root cause since some people with this (or other) auto immune conditions also have this kind of colitis. A stool sample will be able to tell you more.