I'm A Healthy 21 Yr Old Female. No STDs. 1

I'm a healthy 21 yr old female. No STDs. 1 pregnancy/birth in 2010. I have a history of bladder infections, and every test I've ever had done has always come back saying I have some sort of infection, until now. All my cultures were negative for any kind of bacterial infection. My bladder and my kidney's hurt. What else could be causing this?