When I had been on Lithium a few years ago, I had no negative side effects. I took Lithobid, so I do not know if that made the difference.
I switched to Lamictal, cause I do not need an anti=depressant while on Lamictal, where the time I was on Lithium, I also needed an AD, and the SSRI's were stopping to work, even at the doses recommened.
I found Lithobid to be excellent for what I needed it for way back then when my personal life was rocky.
Give your doc a call, and let him or her know about this and that you have IBS, maybe their is some alternative or dosage adjustment he or she can make.
Hope you feel better soon.