Celestia is a hard working person. No one will deny this. She may be an all powerful ruler, but even she has her limits on how much stress her body can endure. Luna notices this and decides to help, while also taking the opportunity to explore Celestia's body in way that can only end in a need for the Mature Tag.
Now with audio read by Scarlet Blade. https://www.youtube.com/watch?v=eTQ-Idv-rBc