It seems the people here have the same sort of memory issues as Dory from Finding Nemo, because they have already stated that Aria does not menstruate and as a consequence she can not give birth, besides the point that they have also stated that they, the Vampires, are not so different from humans, so even them may need the help from a medic now and then, that is why Sakuya is a certified nurse, so Aria getting a bout of gastrointestinal problems is not out of ordinary, maybe all that food and alcohol at the ryokan spawned a complete chaos on her gut flora.
Also, yeah, this author has a myriad of fetishes on his H-manga, one of them being anal sex, but I can not remember one occasion of him using farts or shit on an erotic manner, so no need to fear about a scat fetish a la Erect Sawaru, basically this chapter has the same sort of scathological humor as the fight of Kuririn and Bakuterian on Dragon Ball, one of the favorites series of this author.