With a quick recap of the play it is pretty easy to catalogue the trials that Ophelia has been through in the past couple of days. First her father suggested that Hamlet's feelings for Ophelia aren't true and that he may only be using her. Her father tells her to break off all contact with Hamlet, which she does, but she is broken hearted over it because she truly loves Hamlet. Then when she gets a chance to talk to Hamlet, he tells her to get herself to nunnery, that he never loved her, and that she is like all women who flirt and put on their act for men. The conversation is even worse if you consider that a nunnery could be a house for nuns (a convent) or slang for a house of prostitution. She is shocked and hurt by his behavior and convinced that he has lost his mind. She directly states, "O, what a noble mind is hers o'erthrown!"
Later, Hamlet speaks in a very bawdy and suggestive way to her during the play within a play. She probably doesn't know what to make of that -- is she offended? is this more of the old, teasing Hamlet?
Shortly after this it is discovered that Hamlet, the man she loves, has killed her father. He doesn't seem all that remorseful for act either. He has hidden the body and used it as a joking taunt to Claudius.
Ophelia has clearly been through enough emotional turmoil to cause a complete breakdown. The queen probably doesn't want to see her because she knows that Hamlet's actions are the cause of Ophelia's breakdown. She likes Ophelia and seeing this true madness will be heartbreaking and may cause more uncomfortable inward self-reflection for the queen. The queen has already been given a full guilt-trip with Hamlet, and seeing this change in Ophelia will be yet another display of how others act in the face of a huge loss, as opposed to the queen who quickly moved on to a new husband and an almost seamless continuation of her life as she knew it.