Caesar is surrounded by men who are behaving in the most friendly fashion, but Calpurnia's woman's intuition must have made her sense subconsciously that there was something not quite right about their friendliness. She must have picked up subtle exchanges of glances between some of the many visitors. She should have known, of course, that Caesar was not universally beloved, that he had many enemies, and that he had offended many important people. The very fact that everything seemed to be going so smoothly may have warned her subconsciously and intuitively that there was grave danger.
Dreams have a way of telling us truths of which we are not consciously aware. Calpurnia's dream was a prophetic one. Shakespeare based this part of the scene on Plutarch's Life of Julius Caesar, and evidently Caesar's wife actually did have more than one prophetic dream in which she foresaw her husband's assassination. Caesar himself was apprehensive about going to the Capitol that day, but his ambition overruled his own good judgment, his wife's bad dreams, the warnings of the soothsayer who had told him to beware the Ides of March, the findings of the augurers, and all the bizarre phenomena on the streets and in the sky, of which Calpurnia tells him:
When beggars die there are no comets seen;
The heavens themselves blaze forth the death of princes.