My iPhone has started to stop changing to side view?

I have an iPhone 5 that is relatively new, but just today it seems to be acting really weird. When you turn it sideways when looking at photos or videos it's supposed to change the photo that way too, and on certain apps where that feature is available but my phone is physically turned sideways but the screen stays as if the phone were upright? I don't know if there's a certain setting or something that I may have changed accidentally switching it off or something, or if this is just not working? Any help would be much appreciated!

You probably have the portrait orientation lock on. To turn it of when on the home screen swipe from bottom upwards to bring up settings menu. Click the button that looks like a padlock

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