oz, plan out what your actions will be. If you blow it up, how? After blowing it up, what next? Don't react out of emotion. Think logically and have a plan.
I don't care if you two have been naked 24/7 for the last year, she made it clear she wanted privacy to change alone in the bathroom. You pressured on it. She slapped your hand. As R2C said, NOTHING changed. The only thing that changed was you got your feelings hurt. Nothing is different now than it was before you said that the bathroom was big enough for both of you.
Maybe she had to go to the bathroom before changing and didn't want you in there. I've been with my W for 22 1/2years and still don't go to the bathroom (the sit down kind) with her in the bathroom.
Relax, breathe. There is no reason to act. Remember, in DBing sometimes the best action is no action.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018