1. When Gertrude summoned Hamlet to her chamber, it was because she wanted to see why his behavior was so strange of late and if he might have some problems that she could help him with. She wanted to talk to him and see what the cause of his weird behavior is. She chastises him due to his unpredictable way of conduct and because he is constantly showing disrespect towards his uncle/stepfather, whereas he scorns her for disrespecting her late husband by remarrying so quickly.
2. Hamlet is even more mad in this scene, believing that his mother has betrayed him, as well as his late father by marrying his father's own brother, such a short period of time after King Hamlet's death. He obviously isn't thinking clearly, he is showing signs of paranoia, extreme anger, very negative feelings, which ultimately resulted in him accidentally killing Polonius, who was hiding in Gertrude's chambers, listening in on their conversation in order to find out the cause of his madness.