The episode you are referring to happens in Act II, Scene 2. Puck has put the love potion on Lysander's eyes while he slept. When Lysander awakes and sees Helena, he falls in love with her. At that point, he tells her how much he loves her.
Because she knows Lysander has been in love with Hermia, Helena believes that Lysander is just making fun of her. She thinks he's doing that because she can't get Demetrius to love her and she figures he's pointing out her "insufficiency." Because of this, she gets mad and scolds him:
Wherefore was I to this keen mockery born?
When at your hands did I deserve this scorn?
Is't not enough, is't not enough, young man,
That I did never, no, nor never can,
Deserve a sweet look from Demetrius' eye,
But you must flout my insufficiency?
Good troth, you do me wrong, good sooth, you do,
In such disdainful manner me to woo.
But fare you well: perforce I must confess
I thought you lord of more true gentleness.
O, that a lady, of one man refused.
Should of another therefore be abused!