Lizzy thought at that point that Darcy did perceive Jane's feelings, which increased her anger prior to his proposal. It's interesting how Lizzy's understanding of others is clouded by her own feelings and perceptions. She sees that Jane loves Bingley and therefor everyone else must. Even a warning from Charlotte Lucas that Jane hides her feelings too well is laughed off by Lizzy.

Her view of Darcy seems to be filtered through her understanding of her father- that intelligent men will scrutinize others looking for faults that they can laugh at. It is true that at first "he looked at her only to criticise" but that was soon converted to admiration. Lizzy interpreted all of his attention as criticism.

Lucas Lodge: "He has a very satirical eye, and if I do not begin by being impertinent myself, I shall soon grow afraid of him."

Netherfield: She could only imagine however, at last, that she drew his notice because there was a something about her more wrong and reprehensible, according to his ideas of right, than in any other person present.

Netherfield: "Oh!" said she, "I heard you before; but I could not immediately determine what to say in reply. You wanted me, I know, to say "Yes," that you might have the pleasure of despising my taste; but I always delight in overthrowing those kind of schemes, and cheating a person of their premeditated contempt. I have therefore made up my mind to tell you that I do not want to dance a reel at all -- and now despise me if you dare."