A few answers:
1). Maharsha (Chiddushei Agaddot): She did not yet actually have niddah - that was after the Beracha; however, Avraham was careful to not only have regular food as Tahor (pure), he also held of the opinion that a lady is impure מעת לעת therefore the angels told Avraham not to eat the bread because she would become a niddah later that day (perhaps this is part of the Bracha) and so Sarah didn't know she was פרס נדה.
2). Maharal (Gur Aryeh): That she stopped having regular cycles (ארח כנשים) the word ארח would mean a regular cycle, but rather like an elderly lady which can happen. And in fact this demonstrated something totally abnormal compared to young women.
3). Me: The Gemara Niddah 21b says א׳׳א פתיחת הקבר בלא דם. Ie: Perhaps Sarah felt her קבר open and even if there was no blood seen - she had held that she is Tamei. So she knew there is a chance or that she is for sure Tamei but still perhaps "impossible" for her to have a child since there was no actual blood - and maybe this is why it says here in Rashi that she does have ארח כנשים because her womb opened with blood - but later the Torah reports, she says she doesn't have ארח כנשים because she didn't realize there was indeed blood. And this is why we are using ארח here vs with רחל it mentions דרך (maybe).
(Baruch She'Kivanti) the Re'eim says something similar, that she felt her body return to having Niddah (ie: heavy limbs etc..) but not blood. So she was careful to keep the laws of Niddah but didn't think should could have a child.
4). Me: Perhaps Sarah had a slight misunderstanding in the Beracha: it says
ויאמר שוב אשוב אליך כָעֵת חיה...ושרה שמעת פתח האהל
Rough Translation: I will return to you like at this living moment... and Sarah heard from the opening of the tent.
Perhaps, Sarah misheard/misunderstood כָעֵת חיה and heard כְעֵת. The difference is that the first is she will have a child like the same moment (ie: next year at that time). However Sarah misheard with a Shevah, and it means, the amount of time it takes to have a child (Rashbam/Radak). If so she understood the Beracha is impossible (even for a young girl) since she just had a נדה. Avraham would separate for approx 2 weeks. The issue: a). A Beracha would be a perfect pregnancy and this would not be a complete pregnancy by definition (since it would need to be the time it takes to birth; however minus two weeks) and so she thought it must be incorrect. Similarly, perhaps she knew Yitzchak was connected to Pesach, and so either he would be conceived or born on Pesach and now both are impossible.
Support: It comments she was at or heard from the opening of the tent, ie: not directly, implying she misheard. And Hashem repeats the Beracha in verse 14 it states explicitly למועד אשוב, meaning that time of year (ie: Pesach) - ie: Hashem clarifies it is the same type of moment (ie: this was mistaken originally) and restates כָעֵת חיה.