This takes place after "Bridle Gossip".

After healing Twilight and her friends of those Poison Joke side effects, Zecora finally gets to be alone in her hut again, just the way she likes it. Most ponies think she just goes out and buys potions and ingredients, but before heading back to her hut, she bought a little something for herself: adult foal supplies, and she tries it out for the first time. It would have gone great… if she wasn't busted by Fluttershy.

Do not read if you don't like diaper fetish, diaper wetting and messing, lesbian shipping, lesbian kissing, and enema.