Ive been taking acetaminophen (cheap tylenol pm) over 2 years now daily..what effects can this cause?

I've been taking from 8-12 pills nightly (I'm having a horrible time going to sleep due to my job schedule bouncing every single week) this is the only way I can sleep now. But I went to ER swearing I had kidney stones (it feels so much like stones I've had before too many times) after doing cat scan they found stones but still in kidneys not ped. So he couldn't explain why I'm hurting so bad. Both sides of back hurt off & on but I came home it's not easing up & I'm now curious if this can be causing my pain? I don't want to go back to ER because they'll think I'm nuts (hell I"m starting to wonder myself) but I can't get relief...it's my back, my sides & my stomach is very upset & also hurting some. What are some side effects? Can this cause my problems?? I'm hurting & not sure why...please help me if you know anything..