Sometimes I'm more fatigued than others, and that's as unpredictable as my UC.
I noticed that you take Xanax... does that make you tired? I'm not sure of its side effects.
My anti anxiety med, Oxazepam, definitely relaxes me and makes me more tired. When I take it with my dinner, I find myself nodding to sleep sitting up a couple of hours later! So I began taking it a couple of hours later with crackers or something lite, and the evening fatigue has improved. The fatigue might have something to do with the timing of when you take yours, too.
That said, UC in general is exhausting, as others have said.