A yeast infection occurs when the normal balance in the vagina changes, and the yeast cells overgrow. Overgrowth has several possible causes. One cause may be a change in hormone levels. For example, your period or pregnancy can change your hormone levels, potentially increasing your risk of having a yeast infection. A second possible cause may be antibiotics.
Antibiotics can reduce the bacteria that keep the yeast in check, allowing the yeast to overgrow.
Overgrowth may also be encouraged by substances which irritate the vaginal area. Some substances can change the vaginal environment and make women more susceptible to a yeast infection. Substances that may irritate the vaginal area include chlorine in swimming pools, nylon underwear, douching and even scented toilet paper. Overgrowth may also be caused by a warm, moist environment, such as that created by tight clothes, nylon underwear or pantyhose with no cotton panel. A fourth cause may be a change in blood sugar.