Why is Ophelia so upset when she enters in Act 2, scene 1.
Act 2 of a Shakespearean tragedy begins what is called the rising action of the plot. Hamlet has decided to appear crazy in order to investigate the words of the ghost, his father, who revealed that Claudius killed him in the garden. Part of this plot means planting the seed of his insanity in Polonius, Ophelia's father.
Good, obedient girls will report to their fathers during this time period. Ophelia enters to report Hamlet's extremely upsetting behavior. Until now, Hamlet has been Ophelia's secret boyfriend, but her father has demanded she quit seeing him. Hamlet has apparently entered her bedroom. This action alone is a cause for disgrace as no man should be in a young woman's bedroom at all, or even in her presence without others present.
First, his appearance frightens her:
Lord Hamlet, with his doublet all unbraced;
No hat upon his head; his stockings foul'd,
Ungarter'd, and down-gyved to his ankle;
Pale as his shirt; his knees knocking each other;
And with a look so piteous in purport
As if he had been loosed out of hell
To speak of horrors—he comes before me (II, i, 75-81).
He is in a disgraceful state of undress and appears to be terrified.
Next, his actions confuse and frighten her:
He took me by the wrist and held me hard;
Then goes he to the length of all his arm;
And, with his other hand thus o'er his brow,
He falls to such perusal of my face
As he would draw it (II, i, 84-88).
He grabs her arm and simply stares at her with great intensity. Later he sighs heavily and walks backwards out of the door.
Ophelia, truly frightened, has no choice but to report this to her father.