Rotation of an image is stored in the exif data.
When you take an image with your phone in a non upright position the phone will rotate the picture to look like you expect it and write into the exif data.
But the image itself will still be the way it was taken, meaning the data of the image itself is unaltered.

Usually when you send the picture or upload it software will read the exif data and display the image accordingly (rotated). But some sites or some software will ignore or strip the exif data and display the image how it originally looks (non-rotated).