I have this problem. People often assume I'm stupid until they actually listen to me. My college classmates assuming a bureaucratic mistake had been made when it was announced I was graduating a year early magna cum laude; strangers talking to me in a loud, condescending voice like I'm a child. All my life, and so it happens that in casual conversation with people who don't know me, they dismiss anything I say about politics, art, history, etc. People also assume, whenever they find out an interest of mine, that I'm a beginner and they will teach me about it, even though it's usually something I have years of experience in.
It's really frustrating and it's not as if I present as bubbly or timid. People who actually listen to me, respect me. But it's just amazing how many people who've never thought twice about say, insects or the Roman Empire, willl assume that they must know more than me, who's studied it. I think it's partly because I'm short, conventionally feminine and curvy , which a lot of people read as "bimbo" and also because I've been told I have a very open, childlike quality. It's sad how many people think a sneery, cynical personality connotes intelligence and knowledge.
In your case - it could be many things. I've noticed that many people react more to delivery than content, so they could just be dismissing you based on the fact you don't communicate in their accepted way. Also, once people pigeonhole you a certain way, they'll dismiss any input that contradicts that assessment.