Gertude marries, Claudius, her dead husband's brother. Hamlet is less than pleased would be quite an understatement. He is enraged at the idea. He clearly states his contempt for this union in his soliloquy in Act I Scene 2 when he says that "a beast, that wants discouse of reason / Would have mourned longer." This quotation means that he thinks she was disrespectful to his father's memory by marrying so soon (within a month) after his death. He thinks his mother weak for needing a man so soon and expresses this idea in his statement: "Failty, thy name is woman!"
What's more, the thought that she married her brother-in-law sickens Hamlet. He calls it nothing less than incest.
Later, when Horatio comes to tell Hamlet of his father's ghost, Hamlet further reveals his feelings concerning his mother's marriage when, speaking of the hasty wedding,he tells Horatio,"Would I had met my dearest for in heaven / Or ever I had seen that [wedding] day." In simpler terms, he tells Horatio that he would rather see his worst enemy in heaven than see his mother marry his uncle.
So, all in all, I would say that Hamlet is beyond upset at his mother. At this point in the play, he is utterly disgusted by her need for a man--especially since that man is the uncle he despises.