How often does a kidney infection occur without a bladder infection as well? Also there is no bladder pain only on the back and flank?

Kidney pain. It is very possible to get a kidney infection without any bladder symptoms (and these can make you very sick!), or pain can also be from a kidney stone or an injured or inflamed muscle in the area. If you are having sharp pain in your back and/or flank, you should see a doctor for an exam and urine test - especially urgent if you have a fever, too.