I wonder if what you're going through could be described as "morbid thoughts."
If you feel like you have no control over your thoughts, and if the thoughts are "morbid" in nature - -
I would look up Obsessive Compulsive disorders, most people think of people who can't stop washing their hands, but there is a subtype of OCD called "Pure O." Apparently these folks suffer only from the obsession and not any obvious compulsion (ie hand washing).
I hope this isn't what you are describing, because I know how rough it can be. I have posted on this board numerous times about
OCD and the immune system. My layman's hypothesis (I am not a professional) is some type of immune complex or pathogen crossing the blood brain barrier. There is much info on OCD and autoimmune disease if you google.
Don't get me wrong, I'm not qualified to diagnose anything, and there are probably many different things that can go on other than OCD in that area.
PS - do you find things happening like getting songs "stuck" in your head?