My best guess is that was simply a means of feeling out Nanoha's relationship with Chidori in a manner she could readily BS off as an off-colour joke - and going by that rather satisfied-looking smirk she made at Nano's startled reaction whatever hunch made her do it in the first place was amply confirmed.
It's probably telling that when she comes see Chidori again the next day she essentially starts probing for her side of the relationship right out of the gate, which seems somewhat atypical behaviour (Igarashi appears to be one of those people who prefer to talk 'round the actual topic and only come to the point after circling "like cat a bowl of hot porridge", to butcher an old proverb, for a while). She's also rather conspicuously refraining from the affectionate body language she originally used with Chidori both from first meeting way back when and the previous day before the meddling kidsNanoha & potential new club recruits barged in (may be her default behaviour with other girls, but hard to tell given the nonexistent sample size we have of her interactions with ladies not called Chidori) and instead almost seems to make a point of staying at an arm's length.
If I were to make a guess she put whatever plans she had on hold until she could get a better idea of Chidori's specific relationship situation; would imagine the last thing she wanted to do was repeating her previous fatal error of blundering into ill-advised decisions without a full grasp of the context and what the Hell she's actually doing.
That pretty well sums up why I think she did it.
Also, I don't think her shock at the end is disbelief or even surprise that two girls can have a relationship. I think it's surprise that -these- two girls have a relationship. Basically she's processing the fact that Chidori and Nanoha have actually made it work, and that she's missed her shot. Remember my read is that she wants to win Chidori back, and while she suspects something is up with Nanoha, she's still -HOPING- it's just a crush, or one-sided, or ambiguous. When Chidori confirms it's a serious, actual romantic relationship, she gives up, apologizes for causing trouble, and gives one last little lie to gloss over any potential drama and seems to leave pretty gracefully.
I can see where the hostility comes from: She fits a lot of the user-sempai tropes that you see so often in these stories. But she doesn't have that predatory edge, and she just looks sad so much of the time, she feels to me more like someone that knows she messed up and wants to take it back rather than someone who doesn't care about anything but her own gratification.
.jpg?1565629841)
