That exchange on page 19 is really... odd, though.
K: It just means you're really falling in love with him.
R: Is this what love is?
K: I think so?
R: Then I just can't understand. I've searched so many times for a reason, and now I feel gross unable to find one.
Maybe it's the translation, but what is she talking about in that last time? A reason... she's in love with Reiichi? Why would she 'search so many times' for a reason that she loves him, when she was just now led to the conclusion she loves him?
I think Kaoru made an incorrect assumption here. She assumes Risako is jealous due to Kaoru spending close time with her boyfriend in the past, when the reason she's actually jealous is that someone other than her was close to Kaoru.
So, "Is this what love is?" is Risako questioning her own feelings toward Kaoru.
"Then I just can't understand. I've searched so many times for a reason, and now I feel gross unable to find one."
Shows that until now, Risako didn't identify her feelings toward Kaoru as romantic love. It also shows that said emotions confused her and she constantly questioned what she was feeling toward Kaoru. And she doesn't seem too happy with the revelation and wishes she could keep making up excuses for her feelings. Or at least that's how the phrasing makes it seem to come across.