D.B.
I can echo what Rachel H and Amy P said. Anything that throws off the balance of bacteria and yeast in the body, particularly the gastrointestinal tract, can cause an overgrowth. Also, using baby powder in the diaper area can cause yeast - it looks like a diaper rash but it is specifically yeast. So don't use this, and when you change your daughter, try to let her bottom air out a little before you put on the new diaper. If she's an active child and won't sit still to air dry, you can try a blowdryer set on low as long as the noise doesn't bother her.
The thrush treatment may prevent the diaper rash at this point, but I think the doctor was just giving you a head's up that it could occur.