I just watched a movie in the theater with my grandma and I feel like I'm dissociating. This happens a lot of times after I see movies in the theater.

Why does this happen?

Movies can be viewed as a form of hypnosis. Your mind is taken out of the real world and your conscious awareness is taken into a fictional world where you, the personality you are in real life, is no longer present. They are written and designed to cause you to associate with the protagonist or the main character for the duration of the film for emotional impact. This is fine for most people without DIDs, but it can mess with those minds who have the disorders.

It's the same thing with certain video games.

Pretty much this. Also, 3D movies may temporary mess up your balance, which can add to what shapeshifter said. Talking from personal experience

Yes it was a 3D movie so that could've played into it. Thanks for your help

That makes a lot of sense, thanks!

I agree with Shapeshifter and am glad you found that helpful.
What you describe sounds quite 'normal' to me with some films, and theatre. Not something to worry about DankMemes.