I don't want to give up my room for a week when my grandmother visits
What happened last time when your grandma came to visit and she stayed at your room?
I stayed at my girlfriend's house
It's expected that you would want your grandmother to be comfortable. You should allow your grandmother to have your room for her temporary visit.
My grandmother can just sleep in the basement or something. That is my room.
Maybe you can sleep in the living room or share a room with someone else in the home while your grandmother comes to visit. However, you should always try to make your  grandmother comfortable.