Pretty simple solution...
Hi, I am very prone to yeast infections, I use to get them all the time, now, only when I dont pay attention.
Basically your body gets them more often when your immune system is low because you partied, didnt sleep, stress, you were sick, vacation, etc... something you did that is not normal for your body and unhealthy.
But if on top of this you get on the bike and sweat, and dont clean and let dry right away... there is a problem, bacteria that comes from your body and the exterior starts affecting you.
So, be healthy as possible, use cotton underwear and cotton bike shorts, during the day use skirts to ventilate the area, not tight pants. Take a shower and dry right after you come back from any workout, the beach or being with your husband/boyfriend. Be always clean & dry.
Some girls like you and me have to be more careful than others unfortunately =(
Good luck!
Vero