I do think disorders are thrown around a bit, especially Depression, but as for PTSD I've actually had trouble finding people who know what it is! She may be right that it is thrown around I'm not sure, I have had one friend say he had PTSD because he never had a girlfriend and gets extremely devistated when he's rejected (we're talking fainting here). That is not PTSD. And it really pissed me off when he said it.
As long as this person you are seeing doesn't try to claim you DON'T have PTSD based on 3 sessions and the fact it's a term thrown around a lot, there isn't much to hopefully worry about. If she DOES say that, and it makes you feel angry or uncomfortable (nothing is worse than being diagnosed with something, learning all about it and thinking that's what you have, only to be told none of it is the case at all), is the possibility of seeing someone else available?