Why did Sarah doubt the words of the angel? Didn't she believe that Hashem can do as He pleases (as indeed Hashem answered her)?
Ramban (on Pasuk 15): Although the angels came in the guise of men, Avraham, in his wisdom, recognized them as angels of G-d, Sarah (like the wife of Mano'ach) did not. Perhaps she did not even see them.
Seforno: Of course she believed. The problem was, that she assumed the angel's words were merely a prophetic blessing, which, under the circumstances, she thought, could not possibly materialize. Since to have children at her age was akin to Techi'as ha'Meisim, which could only come about directly from Hashem.
Moshav Zekenim: She heard on Pesach, and laughed six months later, in Tishrei, for no change had occurred. She thought that there is not time to give birth in Nisan, for she did not know that it will be a leap year with an added month. Refer to 17:21:151:1*.