In Act II scene 1, Ophelia rushes in to see her father, clearly extremely upset about what she has just witnessed. As she goes on to explain to her father, Hamlet just entered her chamber with his clothes all unkempt and not dressed properly and with a terrified look on his face. Note how his expression is described by Ophelia:
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.
In addition, he went up to Ophelia and stares at her face intently without saying a word before uttering a very deep and mournful sigh and leaving her chamber without looking where he is going. It is important to remember that this scene comes straight after Act I scene 5, when Hamlet declares his plan of putting on an "antic disposition." It could be argued that this is the first manifestation of that plan.