Unexpected Greetings

Whenever someone says hi (or any other greeting), and I'm not expecting it, my response sounds all high pitched and little-girlish. Then of course, since I'm embarrased at how stupid I sound, I'm afraid to say anything else, which just makes the situation more awkward. Or if I'm in a social situation that is causing me anxiety, I'm focusing so hard on appearing calm, that I'll say the opposite of what I mean (for example: "hi" instead of "bye" or "you're welcome" instead of "thank you"). Does this happen to anyone else?