In Julius Caesar, why is Portia upset with her husband?

Portia is upset with Brutus because he has not been telling her what he is involved in.

Portia was a strong woman, and not the most mentally stable.  She was aware that Brutus was up to something, and was unhappy with him because he would not tell her what it was.  He was trying to protect her, but she thought he was shutting her out.

Portia complains that Brutus does not tell her what is...

Portia is upset with Brutus because he has not been telling her what he is involved in.

Portia was a strong woman, and not the most mentally stable.  She was aware that Brutus was up to something, and was unhappy with him because he would not tell her what it was.  He was trying to protect her, but she thought he was shutting her out.

Portia complains that Brutus does not tell her what is going on with him even when she asks him.

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;
I urged you further; then you scratch'd your head,
And too impatiently stamp'd with your foot (Act 2, Scene 1)

Portia asks Brutus if he is sick, since he is up late at night walking about. She can see that he is restless.  She accuses him of not confiding in her because he sees her as a “harlot” and not a wife.

It does not take a genius to be suspicious.  Brutus has a lot of strange visitors, including many of the most influential senators in Rome. They visit him in the middle of the night!  That obviously means that Brutus is involved in something.  She might even assume that he is the leader, since they meet at his house.

Portia does not stay out of it.  She has him followed.

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. (Act 2, Scene 4)

Portia runs into the Soothsayer Artemidorus, who is trying to warn Caesar about the conspiracy.  He does not give her specifics, but it is enough for her.  She is aware that Brutus is involved in a conspiracy to assassinate Caesar, and she is terribly worried about him.

Portia does her best to convince Brutus that she can be trusted and is loyal.  She goes so far, in fact, as to stab herself in the thigh. Brutus promises he will tell her later.  As far as he is concerned, she is a woman and assassination conspiracies are men’s work.  He is probably also a little worried about her stability.  He has a reason to be.  When he is off at war he gets word that she killed herself by swallowing coals rather than betray him.