It’s still not that common, it only occurs in about 30% of pregnancies. And besides the “it wasn’t a dump” line implying that it was blood, the fact that Kiruko expected more bleeding (and cramping/pain) over the next day or two (hence their request for a towel) indicates that it is indeed menstrual bleeding, and not implantation bleeding (and as we see in this chapter they stayed for another 3 days, which is in the normal range of how long a period lasts).
If Ishiguro wanted to foreshadow pregnancy he would likely use and depict a symptom that’s far more common/universal to pregnancy than implantation bleeding, like he previously did with Tokio’s morning sickness.
Still being worried about Kiruko being pregnant after these chapters is like the equivalent of speculating about, for example, if a main character in a futuristic science-fiction story lost an arm after a big battle/fight, because we haven’t seen them remove their cloak since the big fight. But then when the character finally removes their cloak in the most recent chapter/part of the story and shows the audience they are unwounded and literally has a scene dedicated to showing that they have both arms, it would be like someone then going, “ah but what if they have a convincing prosthetic arm, and they actually did lose an arm?”
Yes, that’s technically possible, but it’s extremely unlikely when the most recent scene/chapter/etc. had just been dedicated to showing that character was unwounded. And that’s when this sort of “speculation” gets annoying.
For anyone who might be interested here are some updated character charts for the series