What Causes Recurring Pyelonephritis?

I am i 29 year old female who has had pylenophritis off and on since the age of 6. In and out of hospitals due to the need for stronger antibiotics so Im told. At the age of 13 I underwent a series of tests and was told that I would no longer have this problem however at the age of 16 they came back. I have since had at least 1 a year. They painfull and debilitating and end up dragging myself into a doctor in tears. I suppose my question would be why? Why do I get these so often and since I was so little? How much damage could all this be doing and should I be concerned for my future health. And last could I have somehow passed this on to my two little girls?