When Sarah is told that she is to give birth to a child at the ripe old age of 90, she laughs saying 'my husband is old,' and HaShem seems displeased with such a reaction. He questions Avraham, saying 'why did she laugh...is anything too wondrous for HaShem' (18;12-14).
The difficulty is that at the end of last week's sedra, Avraham also laughed when he was told that he was going to have a child (17;17) and HaShem does not get angry at all. Why this discrepancy? Perhaps an answer is that these were different types of laughter. Avraham's laughter was one of relief and happiness - full of beleif. But that of Sarah was - to a certain extent - one of disbeleif as if she did not really believe that she could give birth to a child at her stage in life.
Perhaps this is why we are told that though Avraham laughed out loud, Sarah's laughter was 'hidden' - she laughed to herself, because she wanted to disguise this small lack of beleif.