That's when Asuka confessed to her and she realized her daughter's feelings, right?
I was actually referring to when Asuka wheedled her way into sharing her mother's bed the first time (at least, first time for high school). Ayako was worried that Asuka was being too clingy, though she did a 180 when given a scare about them growing apart as Asuka grows up.
As for the "delicious" bento incident - we know from having seen Asuka's pov that she was massively (and fairly obviously) "worked up" at the time, getting a sexual vibe off her in the circumstances could have been purely objective. It speaks volumes for Ayako's innocence that it never even occurred to her that Asuka was turned on, let alone that she was the cause.
After the confession - she first tried to convince herself it was a prank, then when Asuka confirmed it, she held together long enough to be supportive. But after Asuka was in bed with the fever we saw that she was actually pretty freaked out. They managed to avoid talking about it until they ended up in bed together again. Even the kissing "compromise" seemed like an attempt to bury it and normalise things - she was taken aback by Asuka's enthusiasm & intensity, and subtly punished her by not waking her up, then was flustered by the smooth "goodbye kiss" the next morning.
I'd point to the kissing as where Ayako's feelings started changing, since after that is where she seems to start surprising herself (eg spontaneously kissing Asuka for comfort after Akira leaves).