Question

Why is Penelope so cautious when confronted by Odysseus after the massacre?

Answer 1

At the begining she believed that he is another man fighting for her hand, stronger than the others but after he told her who he is
She does not want to be deceived.

Answer 2

She though at first that it could be an impostor

Step-by-step explanation:

She had many suitors and many impersonators of Odysseus came to her house. She just wasn't sure that the actual Odysseus was really himself and therefore, acted differently than the usual towards him until she found out that he really was her husband.