Doctors, nurses, or anyone who knows about back pain. I keep getting misdiagnosed.?

I have extreme lower back pain. I have been to three doctors(2 were from a med-express clinic). One said it was a kidney infection and gave me antiobiotics and vicodin. When the culture came back it was negative for any infection. The second doc. said it may be my S.I. joint or something and said to see my G.P for a second opinion. She said it's scoliosis. I had an x-ray and an MRI. Both show no signs of scoleosis and everything is normal. I have also had bloodwork done for my kidneys and they're fine. Can anyone help me figure out what this is or give me advice? The pain gets so bad at night I can't sleep. Nothing helps I've taken vicodin, IBprofen, Motrin, Tylenol.