Portia was suspicious of Brutus and tried to prove her loyalty by stabbing herself.
Portia noticed that her husband was sneaking around, having a lot of visitors late at night, and acting worried. She was not sure exactly what was going on, but she was suspicious. She wanted him to let her in on his secret.
Brutus has been trying to avoid telling his wife that he is involved in a conspiracy to assassinate Julius Caesar.
You've ungently, Brutus,
Stole from my bed: and yesternight, at supper,
You suddenly arose, and walk'd about,
Musing and sighing, with your arms across,
And when I ask'd you what the matter was,
You stared upon me with ungentle looks … (Act 2, Scene 1)
She asks him specifically about the visitors he has had late at night that hide their faces. He is up to something, and she knows it. She feels like she is strong enough to know what it is, because she is Cato's daughter. Cato was one of the senators Caesar fought in the civil war with Pompey.
Brutus feels like he can’t tell her, probably because Portia is unstable. She wants him to tell her and to think that he can trust her, but he has reservations. Anyone involved in the conspiracy or knowing about it would be in very big trouble.
Portia gets angry with Brutus, saying he tells her no more than he would a harlot. He is not treating her like a wife. She asks him if he is sick, and he does not tell her what is wrong.
To prove her loyalty to him, Portia cuts herself. It is a gesture that just makes her seem crazier to most of us. Brutus promises to tell her later, but Portia is not convinced. She has him followed by his servant, Lucius.
Yes, bring me word, boy, if thy lord look well,
For he went sickly forth: and take good note
What Caesar doth, what suitors press to him.
Hark, boy! what noise is that? (Act 2, Scene 4)
From Artemidorus, Portia gets confirmation that her husband is going to try to attack Caesar. This has her worried, because she knows how risky it is. Even though the soothsayer does not tell her directly what is going on, he tells her that he needs to talk to Caesar, clearly to warn him. That is enough for Portia. She secretly wishes Brutus luck in his “enterprise.”
By this point, Portia has figured out pretty much everything. She knows that they will approach Caesar with a “suit” that he will not grant, and use it to arrange his attack. She can’t help her husband any more, but she is happy to know what is going on. She is also very worried about her husband and whether he will succeed.
Portia is a strong woman character. She is bold and ruthless, and also cunning. Like Calpurnia, she is not willing to just sit back and let the men decide things. Portia wants to know what her husband is up to.