Change your toothpaste to an SLS free one. I used to get canker sores every month, switched to SLS free toothpaste (toms of Maine) and haven't had one since. My DD also had constant canker sores when she was 3-4 years old. Switched her to SLS free toothpaste and she hasn't had another one since (3+ years now). Some people have an SLS sensitivity.