Abdominal&back pain. UA RESULTS:blood (small), 27 RBCs, 3 WBCs, 3Renal epi cells, Mucus (rare). LABS: Sodium 134, est. Creatinine clearance 80. Causes?

Kidney. You have Blood in Your urine. Small kidney stones and kidney disease/infection would be my first concern. Drink lots of fluids and go to your doctor and get evaluated.
Here are some ... Just 27 RBCs alone (if urine was collected properly) warrants a thorough evaluation with imaging studies, preferably with CT.IVP and cystoscopy to make sure of no significant concerns such as stone, tumor, etc. Besides, one needs to see why your sodium at 134; so share more health background with Doc to deduce its clinical significance. Otherwise, your kidneys seem working well.