when you have back pain during a kidney infection?

I had one once - the pain is constant, nagging and deep... Feels a lot like you pulled a muscle in your back.
I suggest you go see a doctor ASAP as kidney infections can lead to blood poisoning.

Renal (Kidney) pain can be incredibly severe; some of the patients in the most pain i have ever seen have been those with Pyelonephritis (infection of the kidney) and renal stones.

The pain (and this is a barn door presentation) is typically in the loin + back, below where the rib cage finishes and radiates to the groin.

As alredy mentioned, if you have any concerns you should see your doctor, back pain can come on for a great number of reasons, the majority of these will be nothing to worry but some can be much more serious such as renal infection, aortic aneurysm, pancreatitis, hepatitis etc