There's a difference between sensing evil and actually seeing and speaking with one's father's ghost. Hamlet does sense evil before he sees the ghost, though.
"Fie on't! ah, fie! 'tis an unweeded garden
That grows to seed; things rank and gross in nature
Possess it merely. That it should come to this!
But two months dead! Nay, not so much, not two;
So excellent a king, that was, to this,
Hyperion to a satyr; so loving to my mother
That he might not beteem the winds of heaven
Visit her face too roughly. Heaven and earth!" (I.ii.138-145).
Hamlet does think it is odd that his mother would marry his uncle so quickly after his father's burial. He marvels that she cried many tears and put on a good show during the funeral, but she should have mourned her husband's loss a little bit longer. In those days, women usually mourned by wearing black for at least a year. Gertrude and Claudius try to justify their marriage indirectly by speaking about how Hamlet should accept death as a part of life and move on.
All of this, however, is not as unexpected as actually seeing his father's ghost. Then the ghost delivers the truth about his murder and who did it! Consequently, these facts bring reality and the supernatural right to Hamlet's face all at once; that must be quite shocking for anyone.
Furthermore, suspicion that swims around in one's mind is a lot different than actually knowing the truth. Even before seeing the ghost, Hamlet doesn't actually come right out and say his mother might have done something horrible because he merely speculates on her odd behavior after his father's death. When Hamlet's suspicion is finally supported by his father's testimony, all hope is lost in his mother's virtue and that is also traumatizing.