Typically we think/worry about kidney infections & stones.  Back pain +/- fever help differentiate kidney infections from simple bladder infections as both are associated w/painful urination, urgency & frequency.  Both infections & stones can have bloody urine although the pain of stones is typically spastic in nature as the body attempts to move it down the ureter.  Of course, a musculoskeletal injury could also cause back pain although typically not burning. 

Xrays, etc are typically not helpful when looking for kidney & bladder infections.  They're ordered during an evaluation for stones mainly to prove that there's not a complete blockage leading to increased kidney pressure.  CT scans & MRIs are ordered during an evaluation for back pain to prove that nothing is irritating the spinal cord and that no cancer has spread to the spine.  

Blood tests aren't usually helpful in a work up for stones & musculoskeletal injuries except to prove that kidney function hasn't been impaired.  During an infection, blood tests can help document the level of infection and prove the type of infection.  Urine tests are typically more useful for infections.

However, given that your girlfriend's scans & blood tests were negative, I also think about zoster or shingles which is a reactivation of chicken pox.  The pain is often described as burning which proceeds the development of blisters in a dermatomal pattern or stripe corresponding to a nerve root.

Hope this helps.  Good luck!