Mesora: If you don't understand something, many times its because the idea doesn't make sense, like this one. I don't understand it either.
When someone learns, its essential that we don't deviate from what the text says. God wrote the Torah as it is for a reason, that we examine and analyze His words carefully.
The pasuk says openly that God rebuked Sarah for feeling that something was out of His capabilities:
God openly states that Sarah's problem was some disbelief in God's capabilities. It would seem that her disbelief was not in God's ability to do anything, but in God's will to do something for HER. I say this because it assumes less. I don't want to attribute to Sarah more error than warranted, and by saying she didn't believe in God's mastery over the world, it assumes that she never learned this from Avraham. That prospect I find impossible. Avraham taught her all the main ideas of God.
Perhaps since God's response was about the "when", meaning,
He said "At the set time, I (God) will return to you". God
didn't say "Sarah WILL have a child", but God said
"when" she will have it. God is responding to Sarah's doubt, in
which case, it must have been only doubt if God would affect HER. She
never denied God's ability to do what he wills. I think Rashi on pasuk 12
implies this, she only said "will these breasts once again
suckle" , meaning, will I be the object of God's miracle, not that
she denied miracles. She merely felt that He wouldn't do it for HER. So
God responds in kind, and says, "when" she will have the child.
To teach that He will in fact affect her. The only question is when.