Act 3 Scenes 2-4 Act 3: Why does Viola get so shaken up? What is her reasoning for why this would happen?

This is the best I can gather from sc 4. 

Antonio is arrested by officers of the Count, and asks Viola for his purse, mistaking Viola for her brother Sebastian. Antonio is taken aback when Viola will not give him his purse, thinking that she, as Sebastian, is ungrateful for his help; he speaks of rescuing Sebastian from drowning, which lets Viola know that her brother might be alive. Antonio is dragged away, and Viola hopes that what Antonio said is indeed true, and that her brother might have been saved from the wreck.