My Little Pony: Friendship is Magic - Image #271,743

@McMillah
There are a lot of theories but probably the most famous one is from the story called 'Bubbles.'
It's pretty sad but it's a good one.
Read it here: BUBBLES

Or if that's too sad, you can watch a fun parody (though it won't make much sense if you don't know the original)

Any clue about how she got bubbles as a cutie mark?