I think the point of both Vit C and cranberry juice is that there will be lots of excess that gets flushed out via the kidneys, bladder and down the urethra. In other words, your pee will be acidic. This combats infection quite effectively! So in mho, and with the usual warning that I am not a doctor nor have I ever played one on tv, I second those suggestions. If you don't like cranberry juice, there are several types of herbal tea reputed to help. Here in Norway the one most recommended is from a plant called "kjerringrokk". I'm not sure what that plant is called in English. Doesn't really matter, though. The main point is lots of acidic liquids. Any tea or fruit juice will probably do.

Also, do you use any body glide or chamois butter? Avoiding chafing should help, and that's what those do. You may think you don't need it for a 50 minute class, but I sweat more in a 50 minute spin class than in 3 hours on the road, and sweaty shorts can chafe. So maybe try some glider?

Back in my student days when I was having multiple UTIs per year a doc at the uni clinic once asked me was I sexually active. I wasn't at the time, though I had been some months earlier, so she really freaked me out! Was she suggesting I had an STI all those months later? As in, syph? Nope. She wasn't (whew!). She was suggesting a mechanical cause -- lots of juices flowing, the usual bacteria that're always on the body surface (and mostly for good reasons!) and then the mechanics of having s*x sorta pumping them into the bladder where they didn't belong and were causing trouble. Not that her theory was right just then, but it did help to know when I was next in a relationship: It helps to pee after. So I also second that suggestion: Drink enough that you really need to pee after that spin class.

Use only a very mild soap down there. Don't dry your skin out! Yes, you want to get rid of sweat, and you don't want pee droplets going stinky. But pee is actually somewhat disinfectant (as opposed to poo ... which is why we're taught to wipe from front to back), so you don't need to overwash.

And if none of these simple tricks help, then get thee to a doctor. You don't want those UTIs turning into some sort of chronic thing like kidney trouble.