It can take up to ten days Or longer for intra-articular steroid injrctions to have a full effect. Plus, they don't always work. I've had those injections in my wrists, elbows, knees, and shoulders. Sometime they are useful. Sometimes not. If inflammmation is not a main component causing the pain, it won't have as much of an effect.
Either way, it is not like an instant pain relief, other than the short-lived numbing from the anestheic that is mixed with the steroid. Ideally, it improves enough after a few days that you can begin to move the joint better. Then, over time things gradually improve.
Since they've pinned it down as arthritis, I would not worry about it being related to your ileostomy, or elsewhere in your gut. Arthritis does not go away in a few days in mst cases. Generally, you have to learn to deal with it, stretch, and stay active. If you faor the joint too much, you can lose range of motion.
Jan