Halena explains her reaction to Lysander's behavior in her speech in Act II, scene ii. The background is that Halena has just chased Demetrius back into that part of the woods, saying "O, I am out of breath in this fond chase!" [fond: cherished but unlikely to be realized]. Incidentally, this line provides a lively play on words as Helena's two meanings combine. The first meaning is that Halena is fond of (affectionate toward) Demetrius and thus is chasing him. The second is that she holds the cherished idea, which is not not likely to be fulfilled, of restoring a love with Demetrius.
Puck has mistakenly poured the flower nectar in the wrong man's eyes thus anointing the vision of Lysander, not Demetrius, so when Halena awakens him, he pours out expressions of admiration that surprise and wound Halena. As she says herself, she thinks he is mocking her, or treating her with ridicule and contempt. For Halena, this is a double wound. Not only is she rejected by (and run away from) the man she truly loves, Demetrius, the other man insults and derides her. She feels scorned for her "insufficiency," as she describes her inferior beauty and inability to win Demetrius's love.
In summary, Halena does not believe Lysander when he makes verbal gestures of love and admiration toward her such as,
Halena, feeling wounded and humiliated, thinks that Lysander is really being cruel and mocking due to some inexplicable, sudden spite and malice toward her. She exits after saying, "you do me wrong, good sooth, you do," and chastising him for his "disdainful manner":