I don't think nightmares have to be from trauma BBT7; most children have them and lots of adults (myself included). It's true that people with PTSD have them also but that's an add-in :-) I have them reading "scary" books! :-)

But Post "Traumatic" Stress Disorder would have to have a trauma, that being its "purpose." Could it be you have forgotten something particularly scary or abusive in the relationship with your father, Anna? Could also be you have something like GAD, where there's free floating anxiety. http://psychcentral.com/disorders/sx24.htm