When a UTI gets into the kidney, what could happen?

I've been at the lake and river lately, and have noticed burning in my pee, but ignored it. I am only sixteen and have gotten one already. After about a week later, my side started hurting, at first i thought it was a minor side ache, but it only a few days later put me in tremendous pain. I'm not a big water drinker either, i don't know if that makes it worse, or what.. but please let me know if it is something a doctor should see, or if i should rely on water and cranberry juice.