Why do people just say I'm overreacting even though they know I am autistic?

I'm not really sensitive but sometimes people touching my shoulder to tease me or tires screeching or loud thunder do trigger my sensory issues and the loud thunder is understandable but with the other issues a lot of people like my extended family or some schoolmates would say I'm overreacting or OA (local Filipino abbreviation for overacting) and its just so annoying and I dont wanna say I'm autistic because they might discriminate me. Before I told my uncle and he said it was not true because I was born in a sterile environment and I was a straight A student and that also really pisses me off.