Her feelings were brewing long before but that was when things started to come together.
For attentive readers not trying to make every interaction of two female characters something to coom to, the ground work was laid in the lead up to the date that was just the moment she became aware that she had caught feelings.
However, for Kogahara who is simply shy and inexperienced in love and not some sort of A-label, this is very likely to be used as a reason for not addressing her internal dilemma and trying to sort her feelings, and instead as a reason to stomp her feelings back into the box, stay out of lemon's way, and avoid the messy problem of how she feels.
You know, until she realizes it won't work and things boil to a crisis point.
You act like she has a choice.
After all the shit she put Sunao through, turning bi for Kogahara is the LEAST she could do to make it up for it.
(lets just go right ahead and put a nice, clear /s on that because I have a feeling that's probably someone's unironic position)