I'm going to freak out on my grandmother.
Why do you plan on freaking her out?
I need to go to the toilet urgently
I understand how that might make you feel anxious and want to freak out.  It is important to be respectful of your elders, though.  I'm sure she will understand if you don't freak out.
I don't care. I'm not wasting a good chance to make a scene.
You should be more respectful and don't do things to make her life more stressful.