So here is the deal. I have been trying to find a fic where Pinkie pie was a changling long before she came to ponyville. I have tried this site as well as fimfetch and still nothing. I don't remember if it is a romance or any other tags that would be on it. This lack of knowledge is driving me nuts, especially when I know I enjoyed the story and wish to read it again. Here is what I do know. WARNING: SPOILERS AHEAD!

Pinkie pie became friends with a changling as a filly and one day while they were playing she is crushed by some rocks. Her parents find out and accept when the changling chooses to take her place.

Can someone please help me locate this fic?

6481524
The fic you're describing sounds an awful lot like https://www.fimfiction.net/story/31089/the-three-sisters, which featured Rarity in an almost identical circumstance. Are you certain it was Pinkie Pie you were reading about?

6481536
Yes I'm sure. Because it was her being a changling being the explanation as to why she was able to do things no earth pony should be able to do.