Did he ask because i look mexican or to be rude?

unless there was something else to the story, they didn't mean to be rude, but you are right, they definitely made an umption about you - but it wasn't a negative umption, and it turned out to be true. I can understand why it left you with a bad feeling - it is weird to have umptions made about you - I might have teased them a little for making an umption, but it is best to try not to be offended when no one was trying to create negativity