Added notes. I'm not sure if I translated the second panel correctly.

DO NOT GET IT.

Also, I would translate it the same, except that perhaps I'd say "I'm already full". Doesn't make it any less WTF, though.

Joke works with Japanese perverts.
Onaka is used as vagina in most ero-ge, and ippai is of course used for being stuffed full of cock.

(Of course, she really just meant her stomach was full (from eating lunch or whatever))

Right, I should have been able to infer that. Still, the connection between the first and second panel (きれい〜 でも、もうおなかいっぱいだしい, wtf?) kind of eludes me.