Imposter syndrome and Surprised responses

Since being diagnosed as autistic as an adult ( last week) I am suffering with imposter syndrome. I immediately felt relief after the diagnosis and felt a weight had been lifted, so I wanted to tell everyone. However, many people were surprised and have said things like, ' wow, you can't tell,' and 'you're really good at masking.' This just consolidates my fear that I'm a fraud.  I don't know how to respond to it. I haven't been consciously masking. I've just been surviving in the only way I knew how to. I wasn't prepared for the questions that have followed, 'what are your symptoms?' 'what makes you autistic?' I feel like they're asking me what colour underwear I'm wearing!! 

How do people respond to/ deal with this? I feel I'm suddenly off script and I don't have the answers or an explanation.