Well serotonin syndrome won’t cause the urinary problems you are having and that’s your first priority. Some good meds, few side effects, to treat bladder spasms till they figure out what’s going on.
I would be surprised if serotonin syndrome was a problem. Keep in touch ...let me know what they find. Good luck.
Oh by the way low back pain can be your kidneys talking to you. If it’s mainly on one side, that’s the kidney having problems
Go to the above site...probably the best info you can find. Also read the comments below the articleles