Here, whatever haopened clearly does mean something ; probably something bad for the speaker. So his speech is idiomatic (the perhaps unexpected use of the present continuous is often a clue), and he is saying (I'm doing it too :)) to himself that whatever it is, it really doesn't mean anything. "Pretty", here, means, "fairly", or "quite", or "somewhat" or "a little" - but not "very" or "completely". So, he is trying to tell himself that something does not matter. But he is not really convincing (or, you might say "fooling") himself.