I have never heard of this and have no explanation. My first guess is that the events are not in fact associated except randomly; there is a very powerful psychology that attributes adverse events to whatever seemed to precede them. Before I accepted the assertion, if you were my patient I would ask you and your wife to keep a written diary of the exact times of fellatio and onset of canker sores for at least 2-3 months--then would analyze the diary before assuming it isn't random. Another useful approach might be to use condoms consistently for oral sex for a couple of months and see if the canker sores continue.
Sorry I can't be more helpful. Good luck-- HHH, MD
She is probably inadvertently biting her inner cheek/lip and/or the abrasion of the teeth against the cheek/lip is cutting the skin, causing canker sores. People who are prone to canker sores get them almost every time they have trauma to the inside of their mouths.
The Content on this Site is presented in a summary fashion, and is intended to be used for educational and entertainment purposes only. It is not intended to be and should not be interpreted as medical advice or a diagnosis of any health or fitness problem, condition or disease; or a recommendation for a specific test, doctor, care provider, procedure, treatment plan, product, or course of action. MedHelp is not a medical or healthcare provider and your use of this Site does not create a doctor / patient relationship. We disclaim all responsibility for the professional qualifications and licensing of, and services provided by, any physician or other health providers posting on or otherwise referred to on this Site and/or any Third Party Site. Never disregard the medical advice of your physician or health professional, or delay in seeking such advice, because of something you read on this Site. We offer this Site AS IS and without any warranties. By using this Site you agree to the following Terms and Conditions. If you think you may have a medical emergency, call your physician or 911 immediately.