I agree with what the other people have said. Definitely get rid of any bubble bath. I'd try having her just soak without any soap on her body at all. Whatever bath soap you're using, try something else, preferably unscented. Choose a "free" (unscented) laundry detergent. Also, if she's still in diapers or pullups, try other brands. If she's potty trained, have her wear a nightgown, no underwear to bed.
I've had problems with UTI's all through my adulthood and some people are just wired for them. But I think you should see a urologist to confirm. Make sure she finishes all antibiotics. If the UTIs are spaced close together, she may have an antibiotic-resistant strain that isn't quite clearing up between infections. If she goes to daycare, make sure they know to send her to the potty every couple of hours and that she needs access to water all day not just at snack time. Also, teach her to sit on the potty and wait to be completely done peeing - kids can be in a hurry and rush off to get back to play, leaving some urine in the bladder.
I also wanted to add that it's not unusual for girls to have UTI's, at all. Women get them more than men, and it's very common in girls. Which doesn't mean you shouldn't take it seriously and see a urologist, but I also wouldn't be overly alarmed.