"Snap" impression/judgement is an impression or judgement made straight away, and suggests that the facts weren't really considered. So in this case, because they'd been there 9 months, they could make an impression based on knowledge and facts, so it couldn't be a hurried "snap" impression.
"Maybe I was just feeling self conscious". - You're right, self conscious means to be aware of yourself. In this case, because the writer sees beautiful people, they are more aware of themselves maybe not being as beautiful!
Hope that helps :)