Like you, I think this part of Elizabeth's evolution in feelings, shows she was first attracted to Mr Darcy, though she early, violently and steadily checked this attraction, as no girl with a normal self-esteem could accept an attraction to a man who slighted her so. This is why she overreacted.
When reading his letter, this checking was maybe over, but there was another, a double one : her shame of herself, that made her fear seing him; and her believing he should consider her his worst ennemy. With these two feeling, how could she allow herself to entertain a liking of him? A new check was necessary, but though double it was easier to remove than the first. Of course, we can find that she is not so clever, believing so after the letter he had written... so maybe it was the only work of the forst part of this check (shame).
At Pemberley, Mr Darcy shew her that she had been wrong in her appreciation of his feelings, so the check was unblocked and she could let herself go to her initial attraction (chapter 44).
Of course she had motives for loving him : we are told they are respect, esteem and gratitude.
But at first she had only the symptom of her initial attraction : "Elizabeth was pleased, though, when she asked herself the reason, she had very little to say in reply."
That is, she was self-pushed to him, eager to know more of him and to be in his presence. Which is very probably what she was starting to feel when she first met him. She would probably not have fallen so rapidly in love with him, because she didn't know him well enough, particularly in anger, jealousy, forbearance... and at his own home too, as it has been often said that Pemberley harmony reflected its owner's; and because at Pemberley, he was so deeply and apparently in love with her, that she needed not to be careful not to let her love grow quicker than his.
I do believe that, within not so many hours after the chapter 44 questions, she was already loving him, though not having been able yet to get sure of her love. In these conditions, I don't find this so surprising. And, though her mind was not sure of her love yet, she had no hesitation to tell him everything about Lydia's elopement, because he was already the rock in whom she confided. Even a very independant woman wants a steady, reliable husband... and her feelings were already there, when her mind was still behind.