'Flustrated' looked to me like a potentially useful portmanteau word, and at least Merriam-Webster seems to agree, which would give some legitimacy to 'flustrating'. Whether the person you hear saying the latter had this in mind is, of course, another matter.

To give an idea of how I see it as potentially useful, there are some frustrating events which leave a person in no doubt that there's nothing they can do to remedy the situation (or that they have no choice but to put a lot of work into fixing a situation which never should have arisen), while others might leave a person in a tizzy over what to do now.