Before anyone says that this change of heart is forced, or too fast, or bs...it's not. Or at least, it's possible
"But why she never thinked about her friend in a romantic way, then?"
First of all, she "did". Her days and thoughts were always full of her friend. Senpai quickly noticed it, you see.
Various things could have prevented it "becoming" love in the first place: it could have been a felling of abnormality (she dated only boys and never gave a thought about same sex relationships...and obviously, society's pov on the matter), she could very well just don't understand it and just mistook it for a deep friendship, or she couldn't picture her friend as someone that liked girls and so the thought never crossed her mind. And whatever in between, obviously.
But care and affection were already there
So, what changed? Obviously the status quo.
Her friend's confession changed everything, from there she could see things differently...more so when the friend tried to be considerated and started distancing herself.
Until then their friendship (and their "being together") was taken from granted, but this changed so fast that she almost couldn't process it.
They were NEVER divided before, and with that she started to "miss her" and understanding even more what her friend meant to her. A pain that she never felt before; peaks of affection that she never noticed before.
You know what they say, people understand the importance of others only when they are not there anymore...something like that
I mean, this doesn't even mean that she is a lesbian. She could be bi, she could still prefer man but loving her as the only woman that she can fall in love with.
Does it really matter? Nope