If you think you are too small and insignificant to make a difference, try spending the night with a mosquito—African proverb 

When Applejack came to Fluttershy asking for help, Fluttershy immediately agreed to help her without asking too many questions. She didn't expect to be thrown into a barn. Or to meet a cow. Or to discover a friendship was in danger of ending forever. Or to become determined to figure out why, so she could find a way to save it...

Actually, she didn't expect most of what ended up happening.

Warning: contains dairy cow.

Takes place in the middle of Season 7, sometime after The Perfect Pear but before School Daze. Can't give you a better timeline then that.

My Little Pony: Friendship Is Magic is owned by Hasbro. So is Littlest Pet Shop, but that's not important to the story so you can forget about it if you want to.