"Seems, madam! Nay, it is. I know not" Why is Hamlet offended by the word "seems"?

Gertrude asks why Hamlet is still mourning his father for so long, Why seems it so particular with thee?

Hamlet turns Gertrude's comment back at her. He says that he doesn't seem sad, he actually is sad. Hamlet implies that his mother really is not upset Hamlet Sr's. death.