I'm not sure where in Poland her birth certificate says, but she was born during or before WWII. All I really know is when she was a young girl her family escaped from East to West Berlin.
So, after WWI Germany lost some of their land to Poland, right? I'm thinking maybe she was born in that region and after the invasion of Poland they moved to Berlin? I don't know enough about the World Wars to make a guess.
Yes, Germany did lose land to Poland after 1945, see this map. The yellow areas in the map were the pre-1939 borders of Germany.
Note that there were a number of ethnic Germans who lived in interwar Poland, as seen in this 1931 language census. The densest German population was in the so-called Danzig corridor which the Allies ceded to Poland after the First World War to give the newly independent Poland access to the sea. The German minority in Poland are more better known by the term Volksdeutsche and it is also possible that your grandmother hailed from this group.
hi! hopefully someone will drop by to help you out, but meanwhile, I found a couple of posts that might give you a bit of background
Before WWII there was a sizable minority of Germans in some regions of western Poland. These include: Pomerania, Silesia, and Greater Poland. Following the Napoleonic Wars these regions came under Prussian, and then eventually German control. Germans have settled, and been granted towns and cities in those regions since the Middle Ages, so your grandma likley came from the German minority in Poland.
During WWII the Nazis made an effort, with propaganda, to claim that western Poland was actually German. This may be why your grandmother claims she's from Germany.
