You are correct to note that Portia appears to be worried or nervous. Shakespeare does a great job, through the text she speaks, of demonstrating this:
- She tells Lucius to run quickly to the Senate and chides him for hanging around, but he reminds her that he hasn't left because she hasn't told him what he's going there for.
- She asks to be given "constancy," which she considers a manly attribute and comments on "[h]ow hard it is for women to keep counsel," which means she's finding it hard to keep a secret.
She finally instructs Lucius in his errand, which is to find out how Brutus looks and to "take good note/What Caesar doth, what suitors press to him" and report what he sees back to her.
And, from her request, we can surmise that, even though it is not a scene played out onstage, Brutus has given in to her pleas from Act II, scene i to take her into his confidence and tell him what is on his mind. Therefore, she is privy to the plans of the Conspirators, which is to murder Caesar. This would be the overriding reason for her to be "worried" and "nervous" in this scene, for her husband is about to commit treason against Rome.
The fact that Portia is in on the plot is further supported by the questions she asks the Soothsayer and what she says in her final speech of the scene:
. . .O Brutus,
The heavens speed thee in thine enterprise!
. . .Run, Lucius, and commend me to my lord;
Say I am merry. Come to me again,
And bring me word what he doth say to thee.
So, by the end of the scene, she confirms that she knows about the conspiracy, but she does not tell Lucius. Surely it can be assumed that she does not tell him because she promised Brutus that she would not tell anyone. It is also not hard to imagine how important secrecy is to the potential success of Brutus' plan, another reason for not telling the servant.
So, she only asks, initially, for Lucius to find out how Brutus looks and what happens to Caesar, but then, at the end of the scene, she also wants Lucius to also send Brutus her love and support and come back and tell her what he says.
For more on Portia and this scene, please follow the links below.