It may be due to weakened immune system. Vaginal yeast infections are caused by the same fungus that causes oral thrush. Thrush is more likely to spread to other parts of your body, including your digestive tract, lungs and liver. So consult doctor you may need antifungal medication.
Eat yogurt and acidophilus. They don't destroy the fungus, but they can help restore the normal bacterial flora in your body.
Practice good oral hygiene. Try warm saltwater rinses.
Reduce the amount of sugar and yeast-containing foods. These may encourage the growth of candida.