What prompted Hagar to belittle her mistress Sarah?
Rashi: When she became pregnant from the first Bi'ah, unusual in itself, she claimed that, clearly, Sarah was not the Tzadekes she made herself out to be, bearing in mind that, in all these years, she did not bear children, whereas she (Hagar) became pregnant from the first Bi'ah. 1
Riva: It is assumed that a woman cannot become pregnant from the first Bi'ah! Actually, Tana'im argue about this. (Rashi explains according to the other opinion.)
QUESTIONS ON RASHI
Rashi writes: "And he came unto Hagar, and she conceived - from their initial relations." Why interpret thus?
Gur Aryeh: Otherwise, why add, "And he came unto her"? [If it was to teach that Avraham acquiesced to Sarah's request, the verse should emphasize his role, "And Avraham came 1 etc."] Rather, the verse puts "he came" next to "she conceived," to show that she conceived from their initial relations.
Rashi writes: "Her mistress was lessened in her eyes - [Hagar] said, 'This Sarai, her private life must not be how she is perceived in public!'" Why explain thus, perhaps it was simply due to the fact that Hagar had conceived, which would make her beloved to Avraham?
Gur Aryeh #1: If the pregnancy itself was the cause of her change in her attitude, the verse should say, "she saw that she was pregnant." It in fact uses the past tense, "she saw that she had become pregnant" [i.e. immediately], and she made aspersions about Sarah's righteousness.
Gur Aryeh #2: The word "and she saw" is extra. Here, it means "she understood" the situation in a way that caused Sarah to be lessened in her eyes.