I fell backwards down approx. 6 stairs. there's a 2x6 in abrasion on my left flank which is fine. but there's a deep pain in my left lower back. there's no blood in my urine. it's a dull pain w/sharp pains upon movement. do i need a hospital or no?

You more than likely sustained a strain and contusion to your low back. If you're able to ambulate with no leg limitations, I think you could wait &see your physician at the office tomorrow rather than heading to the emergency room tonight. Apply cold to the painful area for 20 minutes a few times& ambulate a bit without stressing your lower spine. Strict bed rest isn't advisable unless necessary