ok apple has natural pectin in it as well as fiber that's probably the cause if its not normally fed that frequently.
Watermelon, cucumber even some citrus like oranges to help loosen up her poo to make it easier to go, yogurt with live culture as well will help flush her system and give her some good bacteria in her stomach. Skip any "treats" n just offer her these until she poos
Monitor her closely if she still doesn't poo take her in to the vet