What Causes Severe Pain In The Lower Back?

I have suffered from lower back pain for years midway up the back to the left of the vertebrae. in the past two weeks, I have experienced severe pain particularly when prone or lying on my back at bed time. I ve been to my internist who ordered a ultra sound focused on primary organs, pancreas, kidneys, gallbladder, etc. results were unremarkable for anything wrong. I ve been taking advil throughout this past two weeks mostly at bed time. When the pain was at its worst, I could literally feel it pulsating in my back area. My internist has not ordered any next steps since the pain has ebbed and flowed for past 4-5 days. I am wondering what type of specialist I should go see because I at least want to know what is the cause of the pain which has not totally subsided. thanks