So Luna was eating and see's Celestia. She wants her to go away but Celestia wanted to know if she gained or lost any weight and as she poked Luna, or, "Puni", Luna had a very tender body. So Celestia was playing around with Luna, poking her, making her laugh a lot from such a tender body she has and Celestia finally stops and finds how Luna is cute with such a tender body. And of course, Luna gets really hot and sweats from all the poking and ends up crying of laughter.
I was wondering what "PUNI" is supposed to mean, but then I remembered something:
The Japenese tend to use onomatopoeia in regards to their portrayal of sound effects in comics. "PUNI" could be construed as something similar to the squeaky sound you'd hear when you squeeze a rubber toy.
"PU" (pronounced "poo") would be the initial squish when you squeeze it and "NI" (pronounced "nee") would be the sound you hear when you release the pressure of the squeezed toy.
tl;dr Luna is supposed to sound like a rubber ducky when Celestia pokes her. To which I say "OHGODITSSOADORABLE" and proceed to have a cuteness-induced heart-attack.