-Lavan searched all the tents for his missing idols.
-Rachel who had stolen them placed them under the saddle and sat on them. She then excused herself from getting up by claiming she was in midst of her period (Niddah) and was unable to stand.
-This was not so far from the truth, for Rebbe Akiva tells us that Avodah Zarah causes impurity similar to that of a Niddah.

(Rabbanu Eliyah of Vilnah - Kol Eliyaho Vyetzeh)