Chris K.
                    
                    
                    
                    
                    
                    
                    
                        
                        
                        I've found that toothpastes that are labeled as teeth "whiteners" can give me canker sores.  As soon as I switched to a different toothpaste, the sore went away.  I'm not sure which ingredient in the whitening toothpaste causes the problem - it may be baking soda.