East or West Germany? West Germany had many different methods to East Germany (I'm sure you know East Germany was Communist/Russian). If you returned in 1991, you were indeed a WWII soul, but where, only you can know that. Most souls who returned (reincarnate/born) between the late 1940's up until around 1992, were ex-WWII/Cold War souls. Germans left Germany in droves (from my own research), post-war because they were often ridiculed for living in Hitler's Germany, so it's not really a surprise to know that you live in Oklahoma now. Where do you 'feel' you're supposed to be? Often we return (reincarnate/born) to a country for a reason, so do you know your reason for returning (reincarnating) to Germany? This might be a subconscious thing that you are not consciously aware of yet, but know subconsciously.
If you left with unfinished business in the former place/country/time, then I understand the feelings of feeling 'out of time'. I left unfinished business behind in Third Reich Germany and German occupied Czechoslovakia, which is why I long to return to these places in this lifetime. I often feel 'foreign' being all the way over here in Australia, because for me - Europe (Germany/Czech Republic) is 'Home'. Covid travel restrictions severely dented my ability to heal and return, so I am hoping when restrictions ease I can continue in my healing.
Birthmarks are often reincarnation-based physical evidence, but many people have birthmarks from various things and therefore cannot always be used as a definite reincarnation marker. I don't feel this has anything to do with your birthmark, but unfinished business from your former life wherever that was and the trauma you sustained possibly from WWII/Cold War or the post-war aftermath.
Eva x