Solubility of glucose
The dissolution of glucose in an aqueous solution is an endothermic process.  As stated in a comment, a reaction need not be exothermic to be spontaneous. Don't forget about the entropy term.  

Simply put, the glucose dissolved endothermically in your saliva causing a heat transfer from your mouth to the glucose/saliva solution, resulting in a slight decrease in the temperature of your mouth, making it cooler than it was prior do dissolving the glucose.