the events of that night are a completely independent story that was never ever brought up again. Even Itsuki herself never learnt of what happened that night. With the ending of this chapter they set themselves up well for it. If they didn't show these events in the next chapter as a flashback, I don't think there would ever be a good time to show them in the manga, unless they deviate from the source. And it'd definitely be a waste if they choose to never adapt that part.
Also, I think the hints of Rin thinking about what Itsuki said about "her true feelings," and the whole panties thing, were already plenty suggestive regarding "something happening." In fact, till I read the story itself, just from those hints, I actually thought the whole "pee" thing was just bogus to cover up how they had sex (or at least became intimate in some way that made Itsuki wet) so Rin can justify washing the panties. It was a twist for me in a different sense

Also that story is in the same volume, just at the end, so it's not really "much" later anyway
