I think nightmares are pretty common with panic attacks, though they're also a common symptom of PTSD. I think it's best at this point not to dwell on the nightmares at all; I think trying to figure them out will only make matters worse. It's just more worry and panic that you don't need.

Try just resigning yourself to the fact that you might have more nightmares, and that you don't know why they are occurring. See if you can take an accepting attitude to that uncertainty, and to the unpleasantness of the nightmares themselves. You might tell yourself that there is nothing you can do about it and that they will go away in their own good time. You could even try writing this down on a note card and repeating it to yourself, and then redirecting yourself to another activity afterward.