I'm going to not let my grandma into the bathroom while I'm using it
Is there some reason that you don't want to let her in?
I just want to pee uninterrupted.
It seems very normal to me that you don't want your grandma in the bathroom while you're using it. It's pretty normal to want privacy there.
That's what I keep telling her.
It can be stressful dealing with family that doesn't respect your boundaries. Hopefully your grandma gets the hint this time.