Yeast infections are caused by the overgrowth of Candida, and that happens when there is imbalance in the body. When the chemicals or hormones in your body are imbalanced it can cause opportunity for the candida to overgrow and stress is a time of hormonal imbalance.
When a person is stressed your adrenals create a hormone called cortisol and it gets pumped around the body and acts as an anti-inflammatory. It is there to ensure organs and muscles get the nutrients, proteins and sugars they need even during times of stress. To balance the cortisol another chemical is released called Dhea. All of this weakens the immune system and causes sugar levels to rise, plus Cortisol can kill good bacteria. The candida is able to overgrow because the immune system is not working at full strength, the good bacteria are not at high enough levels to keep it under control and because it feeds on sugar.