Rarity made new sports outfits for all her friends and challenged them all to a race. However, due to a downpour earlier, the dirt road was turned to mud. Rarity was a little reluctant, but reminded herself how she had once covered herself in mud to get back with Sweetie Belle and boldly decided to run the race. Rainbow Dash and Applejack gave the others a headstart, and when they ran by Rarity, they splashed her with mud, covering her entirely. This is Rarity right after being splashed. But she shook it off and finished the race.

Rainbow and Applejack chuckled nervously and apologized, but Rarity waved her hoof lightly, sending a bit of mud flying, and assured them it was all right, as they weren't much cleaner than her and she could look forward to a nice relaxing bath in her sports outfit, which needed to be cleaned as much as she did, followed by a trip to the spa.

Funny, the Rarity image I traced was from Sisterhooves Social, the episode where she gets covered in mud similar to this.

If you're curious as to the coloring scheme of her sports outfit, think her Equestria Girls counterpart.