Why does Portia send Lucius to the capital? What is she really worrying about?

In Act 2, Scene 4 of Julius Caesar, Portia dispatches Lucius, an errand-boy, to the Senate.
At first, Lucius is confused because Portia doesn't give him a specific errand; she simply asks him to go to the Senate. This is supposed to indicate Portia's disordered state of mind. It also confirms that Portia is still very disturbed by the conversation she had with Brutus the night before (Act 2, Scene 1). Although she doesn't know exactly what is going on, she suspects it is very bad indeed.
Eventually, Portia tells Lucius to "bring me word, boy, if thy lord look well" (line 15) and to "take good note/What Caesar doth, what suitors press to him" (lines 16-17). She clearly suspects that Brutus might take action against Caesar, and this is her method of trying to surreptitiously gain more information. Once again, her distress is indicated by the fact that she hears "a bustling rumor like a fray/And the wind brings it from the Capitol" (lines 21-22), but Lucius hears nothing.
Before Lucius leaves, the Soothsayer enters the scene. When he speaks of wanting to befriend Caesar, Portia's panic is heightened, and she asks if someone means to harm Caesar. The Soothsayer's reply only seems to further her suspicion of what Brutus means to do. When she thinks she is alone, she says, "O Brutus/The heavens speed thee in thine enterprise" (lines 46-7).
She immediately worries that Lucius might have heard her, so she makes up a cover story about Brutus and Caesar being involved in a lawsuit. She revises her instructions to Lucius one final time.

Run, Lucius, and commend me to my lord.Say I am merry. Come to me againAnd bring me word what he doth say to thee. (lines 51-3)

Portia's lie—that she is "merry"—shows that she is a dutiful Roman wife, willing to pretend to stoicism and calm, even when she is anxious, in order not to upset her husband.
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