In Act 3, sc. 2, Hermia discovers that Lysander professes to love Helena.  He says he no longer loves, and even dispises, Hermia.  She is dumbfounded. She says, "You speak not as you think. It cannot be."  This tells us that she was sure of Lysander's love for her.  Apparently Lysander had convinced her without doubt that he loved her before he awoke with the juice of Oberon's magic flower on his eyes.  When Helena accuses Hermia of joining with Lysander and Demetrius to make fun of her by having the two men proclaim love for her, Hermia is again completely perplexed and says, "I am amazed at your words." Continually through the conversation that is going among the four lovers, Hermia asks the essential question of how it is possible that Lysander now professes to love Helena.  Hermia's confusion again attests to the fact that she had felt secure in Lysander's affections.  Then, rather than get angry with Lysander for his changing of affections, she turns on Helena and accuses her of using her feminine wiles, along with her height, to lure Lysander.  This tells us that Hermia does love Lysander.  Hermia doesn't ever stop professing love for him, so she, unlike him and Demetrius, is constant in her love.  When Robin says, "Lord, what fools these mortals be!", he isn't just talking about Bottom and his troupe.  The fickleness of Lysander and Demetrius is meant to show foolishness, too.