A person native to a culture often does not perceive the subtle influence of that culture. That all changes when a missionary enters his host culture. Everything is different, he feels, smells, senses all the differences in an overwhelming cacophony of stimuli.
Duane Elmer gives the following helpful list as symptoms of culture shock in his book, Cross Cultural Connections. If you are going through culture shock this list may not lessen the pain you are feeling right now, but it can help you identify what is really going on.