Ok about a month ago my sides starting hurting, it started burning when I urinated, I felt as if I had to use the restroom every 5 minutes, and the burning got worse. I figure it was probably a bladder infection (which I'd never had before, but my mom used to get them when I was a kid, and I have friends who get them and have described their symptoms to me) I was out of town at the time so I called my doctor and he prescribed me Septra which I took as I was supposed to. It kind of went away but not entirely, because I still had some burning/frequency issues a few days after my last antibiotic pill. I went to my doctor when I got home and they did a urine test and I didn't hear back from them so I assume they didn't find anything. A few days ago I started to get the burning thing again and the frequency but it didn't get too bad really, and then it went away. Now the burning is a back a little bit and my left side hurts a lot. One thing I noticed that could maybe be related is that I drank a fair amount of coke/pepsi a few days ago and then I didn't drink any at all yesterday (when I didn't feel the pain as much), and I drank some today and now it's back.

Does this sound like an infection that was never cured, or something else?

It could be an infection or it could be a stone. There are also other problems that have the same symptoms. Best bet is to keep drinking water and get in to see a doctor. I was treated for infections with antibotics several times even though there would be a negitive test. Some doctors just don't put 2+2 together and think it may be stones. The pain from a small stone can come and go, cause the symptoms that you are having and even cause vomiting. See a doctor soon and let us know how you are feeling.