Wearing cycling shorts is not a cause of yeast infections. However, providing a nice, wet, warm environment for the yeast (which is always present in the body) to flourish will. Yeast infections can also be triggered by hormones (ie around that time of the month) so all maybe coincidental. Bottom line, with or without panties under your shorts, you should change out of wet clothing as soon as possible. That also means making sure everything is dry and clean. Bike shorts should only be worn once than washed. I suggest in a washer (but hang dry - NEVER put shorts in the dryer). Also, watch your diet. Eating lots of yeasty foods, such as bread, will cause the yeast in your body to grow out of control.