Fell down stairs... Injury of this kind requires an examination by a doctor, x-rays/scans as necessary and a close exam of your stomach, back and other areas, a simple urine test to check for blood...however, I strongly believe that sprained or broken ribs with the bruised intercostal muscles are the cause of your breathe-in pains...also, not surprisingly your tail bone (coccyx) may be hurting too...
Kidney. Generally there would be blood in your urine. Difficulty breathing in would suggest chest contusion. Does it hurt to pass urine or to sit on the toilet? One could indicate trauma to your urethra, the other injury to your pelvis. Be checked out to make sure there is no major trauma. All the best.
Could have. Depending on how hard and where in your back you hit, you could have injured a kidney. If injured seriously blood usually appears in the urine but not necessarily enough to make the urine pink or red. More likely you severely bruised or broke one or more ribs. You should go to an urgent care and be checked out today so you know what you have. Follow-up with your own MD as well this week.
MIght have. How does it hurt to go to the bathroom. Do you mean urine or stool? Do you see blood in the urine? That would be the way one might tell if you bruised a kidney. The back pain when you breathe and when you move is either bruised muscle and ribs. Might consider if severe a fx rib. Best wishes.