Hi Summer
Did your doc take a urine sample? You would have other symptoms with a kidney infection, but it could be a bladder infection that is turning into a kidney infection.
It could also be that you pulled a muscle in you back and therefore causing you pain. You don't have to injure yourself to have that happen either. I've actually herniated discs just picking a leaf up off the ground!! I'm not saying you herniated a disc. Just pointing out how easy we can sometimes move the wrong way and have a mysterious pain.
Maybe you can get into see an orthopedic doctor to see if he/she can help!
Best of luck
E