Why did Rachel decline to rise before her father, and why did Lavan accept her excuse?
Ramban: Perhaps she tended to feel exceptionally weak during her period, as some women do - particularly those who have few children, who suffer from a lack of blood (low blood pressure?). 1
Ramban: The early generations, in their wisdom, tended to keep away from Nidos 2 , since they knew that their very breath was damaging, and that even their look was harmful, 3 which explains why they generally stayed at home and why people kept away from them. 4
Oznayim la'Torah (citing the Midrash Rabah): Lavan did not accept her excuse - "Vayechapeis", and he (made her rise and) actually searched underneath the 'Kar ha'Gamal'. "But he did not find them" - because a miracle occurred and they turned into jars - in order to spare Rachel the embarrassment.
Indeed, that is why they are called 'Nidos' (See Rosh Hashanah, 26a).
See Ramban, who elaborates.
And that is also why Lavan declined to answer her.
How could Rachel say "Derech Nashim Li" (I am Nidah? She was pregnant, for she gave birth on the road!)
Tosfos ha'Shalem (2) #1: She lied to him. 1
Tosfos ha'Shalem (2) #2: She became pregnant right after this, and gave birth after six months. 2
Moshav Zekenim, Divrei Eliyahu, Kol Eliyahu: Idolatry is equated to a Nidah - "Tizram Kemo Davah" (Avodah Zarah 47b. Since she was sitting on idolatry; she had Tum'as Nidah with her.)
Moshav Zekenim, Divrei Eliyahu and Kol Eliyahu ask how she could lie. What is difficult? Her justification to steal justifies also lying, and especially after Yakov said "Lo Yichyeh"! (PF)
Why is "Lo" spelled with a Vav and an Alef?