I know how you feel. I hate it when someone does that, but it happens to me a bit differently. Sometimes I want to share something that excited me with my family, and I start talking, they either say "Oh yeah?", or change the subject just because mine doesn't interest them, or simply just give me this face ----->
I now don't share anything with them at all, almost at all. I only nerd out about something scientific that I read, but only to my mother. But I think you're right, maybe your mom gave you a nod or something, that "Hey, I understand". Could it be? Has she done that before?