Commiserations, superflybaby - I'm glad she's recovered so quickly.
My DD had a kidney infection at 13 months after I failed to realise that smelly wee = UTI. It's strange, I'm normally Paranoid Mother, but so little did it register with me that it might be a serious problem (I've never had a UTI either, so wasn't aware of the connection, and I thought she just needed to drink more) that I didn't even google it or ask here.
Then she had a temperature for 2 days that I thought was because of a cold she also had... so I only went to the doctor's on the third day
In her case the ABs started having an effect after 48 hours (i.e. that's when her temperature went down) but I can imagine that with a UTI that's progressed less far it could be quicker.
I read one interesting article on some research that found a correlation between a course of antibiotics for something else (e.g. an ear infection) and toddlers getting a UTI. The theory was that the ABs kill off the 'good' bacteria around the urethral area that usually manage to neutralise the bad e coli etc. ones before an infection takes hold. In my DD's case, she'd finished ABs for an ear infection two weeks previously. The researchers did suggestthat if it were all down to hygiene, babies and toddlers would all get UTIs the whole time because of their nappies...
However, a major contributing factor in our case seems to have been the fact that DD had a labial adhesion (inner labia got stuck together... happens because they don't have enough oestrogen at that age) so bacteria could get trapped easily. And yes, I'd failed to spot that, too.
DD has a scan in October to see if any damage was done - hopefully not. She had an ultrasound during the infection which was OK.
Anyway, congratulations for acting promptly! I wish they made more of UTIs and their symptoms in baby/toddler books - it doesn't seem to be highlighted much.