J. -
I had a somewhat similar experience, but by the time we figured out what was wrong with my daughter she had a full-blown kidney infection and a fever I couldn't control. Because kids shouldn't get bladder infelctions (or kidney infections - an advanced bladder infection), we also had to have some additional tests.
We did our tests at Childrens in St. Paul where they used a cathater to fill my daughter's bladder with liquid while they watched on an x-ray. (She was 1 year old at the time.) They wanted to see if the liquid backed up into her kideys. This is called reflux and there are varying degrees of severity. The tests showed that she did have reflux but not an extreme case.
Typically kids outgrown reflux, but in the mean time bladder and kidney infections need to be prevented because they can be pretty dangerous. In order to prevent infections, my daughter is on a low daily dose of medication. Every year we repeat the tests to make sure that the reflux hasn't gotten worse, and see if it has corrected itself. We will monitor this until she's about 6, 7, 8... If she hasn't outgrown it by then there are some very effective surgeries that will fix the problem.
An aside tht you should also know about... If one child has the problem, there is a high possibility that your other kids also have the problem. After having my youngest (now 3)diagnosed we also had our eldest (now 4) tested. Although she has never had a bladder infection, she does has the reflux. Thus, she too is on medication and we are monitoring her reflux every year. This is very managable condition and not entirely uncommon.
Good luck and take care!
A.