MICELOVER,
Welcome to the forum.
How long have you been on the mesalamines? IMO, it could be a build up and a reaction to the mesalamines? Have you asked your doctor about that?
If it's vit. b12 deficiency, that would be an easy fix. Thyroid problems could also be an easy fix, but would require an endocrinologist. Diabetes would require a change in diet. BUT I don't think it's very correct of your doctor to put out 2 possible scenarios without getting all your testing back.
BTW, did he test your blood for vitamin d3 levels?
If your symptoms continue, I would recommend you seek out a Functional Medicine doctor - they specialize in looking at the whole body and the interrelation of various disease states, and IMO would certainly get to the bottom of this puzzle.
I hope you get some answers soon and start to feel better.