Most people have never meet anyone who stutters. And most people equate the speech disorder known as stuttering/stammering with something that everyone does when they're nervous or emotionally distressed. But when people see that you're having trouble saying your name, then they can't find anything to relate it to, in order to make sense of it. So they genuinely think your forgot your name. Most of the time they're not being rude but just clueless or ignorant of what stuttering is or even that it exist. Just laugh it off. It's funny to think someone could arrive at that conclusion anyway. Afterall who forgets their own name? Unless you have a severe case of amnesia..