I have back pain of an unknown origin but mostly normal lab results, any ideas?

[HariKhalsa]

I have severe burning mid-low back pain on my lower left side near my 11th or 12th ribs that has lasted for over a week now. As far as history goes I'm not on any medications, I have no history of trauma. The only thing I could think of that may have caused a severe strain was that I swam a mile at the pool about 2 weeks ago and stretched my back in the sauna afterwards. At first I figured it was just a severe rib strain with segmental joint dysfunction or rib dysfunction so I had it treated with massage and chiropractic care which usually resolves those issues for me, but the pain hasn't changed at all. I've tried icing, NSAIDS, valerian root, and epsom salt baths with no change in pain levels. It hurts most when I move into back extension with left lateral flexion. Some pain with trunk flexion, right lateral flexion is painless.
           I started to worry about potential kidney issues due to the location of pain, so I had a urinalysis and a CBC/Chem panel done: (Normal urinalysis, no blood. CBC/showed 46 alk phos (low) and 3.77 WBC (low)). The pain is worsening. My next step was to probably get a Rib X-ray, and then potentially a kidney panel if the x-ray is inconclusive. The curious thing is that I have no painful urination or change in bowel/bladder habits, the coloration of the urine was normal as well.
         I felt very tired, potentially flu-like on Tuesday and slept all afternoon and all night but have been fine since then. I'm in a committed relationship and have had the same partner for 2 years and tested negative for HIV or any STDs the last time I was tested. It doesn't feel like any strain I've had before, but I'm receiving normal lab results. Anybody have any idea what it could be?