Hello, it might be a good idea to see a dentist, they sometimes know a little more than just a regular GP.
Why do you think you 'got yours from your girlfriend'? They are not communicable. And herpes (if you wonder about that) are easily diagnosed by a doctor (and 80 percent of the population has oral herpes, not a bit deal) and are not typically in mouth on gums or side of cheek. Your doctor didn't 'lie' to you. They do make mistakes from time to time but canker sores aren't hard to diagnose so am guessing it is accurate. Also, are you sure you don't have the name wrong? There is aphthous cankers. I'm guessing that is what is meant. Here's some information. https://my.clevelandclinic.org/health/diseases/10945-canker-sores