Spoilers for more detailed analysis of chapter 45. Let's take those statements as postulates and analyze her behavior from that perspective. First, she expects Chidori to react negatively to her presence, but she feels like she needs to go see her anyways. Maybe she wants to apologize, maybe she wants to see how she's doing, maybe she feels like she might still have a chance. Probably a mix of the three. But she's psychically incapable of admitting to actually desiring another's person's approval or acceptance, or to admitting that she did anything wrong. So she walks in smiling and acting as if everything is normal. She knew Chidori would be upset, and she is prepared for the inevitable "Why are you here?" She actually tells Chidori the truth, but she acts really casual about it and then walks it back when questioned further. She came to see Chidori. Obviously, this tactic is ineffective. Chidori, understandably predisposed to assume the worst about Igarashi, thinks the truth is the lie and the lie is the truth.
So Igarashi changes tack, and goes for a more direct approach. She brings up the past. Remember that time when I asked you out for real and you rejected me? But of course, she can't admit that she was serious or that Chidori's rejection hurt her, so she brings it up in the tone of "What a funny little memory." Obviously this does not have the intended effect, but it does get them closer to the heart of the matter. Igarashi then, awkwardly and terribly, apologizes for hurting Chidori, and tries to recapture how things were between them before that (apparently touching her hair is the only move Igarashi's got. Her hair's a little shorter this time, but that's obviously got nothing to do with me. It worked before, why not try it again?). She's finally worked around to what she was actually trying to say in the first place, painfully starting to apologize (though that half-assed "mistakes were made" might be the closest Igarashi ever gets to a serious apology), and Chidori appears to be opening up to her for the first time since she walked in.
But then she's interrupted. Chidori's back on her guard, and now, even worse, there's an audience. However difficult it was to get even close to opening up to Chidori in private, now that's completely impossible. Plus, oh shit, who's this girl? The Chidori I knew was a shy girl who was guarded with everyone except me. I assumed that she's be sitting in here alone, but now this friend bursts in, and they're obviously really close! So now, on top of everything else, she's (irrationally) jealous. So I think the "ex-girlfriend" thing is her trying to get a reaction out of Nanoha, put her off, and send some kind of signal to Chidori. Basically, some real petty shit, but she was surprised, didn't have much time to think about it. I think she smiles because she assumes Nanoha's reaction means "What do you mean, "girlfriend"? Like, as in dating? But you're both girls!" So there's Nanoha neutralized. See you tomorrow Chidori! Let's continue where we left off.
This really hits home (and more) to how much I've learned to sympathize with Igarashi at the end of this mini arc.
Though I have to admit that I was as apprehensive of her presence as the people who hate her before understanding her why's.
