Genesis 31:35

And she said, Let it not be displeasing in my lord's eyes that I am not able to arise before thee, because I have the way of women. And he searched, but found not the images.

LET NOT MY LORD BE ANGRY THAT I CANNOT RISE UP BEFORE THEE. I do not understand what kind of an apology this is. Do women in that condition not rise or stand? Perhaps she said that her head and limbs feel heavy, and she was sick on account of the menstruation, for such is the customary way among them, and all the more among those such as Rachel, whose birth-giving powers are diminished since they have little blood, and menstruation presses very heavily upon them.
The correct interpretation appears to me to be that in ancient days menstruants kept very isolated for they were ever referred to as niddoth on account of their isolation since they did not approach people and did not speak with them. For the ancients in their wisdom knew that their breath is harmful, their gaze is detrimental and makes a bad impression, as the philosophers have explained. I will yet mention their experiences in this matter. And the menstruants dwelled isolated in tents where no one entered, just as our Rabbis have mentioned in the Beraitha that for some reason had not been included in the Mishnah by Rabbi Yehudah Hanasi. The collection of Beraithoth was compiled by Rabbi Chiyah and Rabbi Oshayah. They are generally found in the Tosephta. which follows the order of the Mishnah. of Tractate Niddah:. See my Hebrew commentary, p. 177, and commencing with the second edition also on p. 548, column 2, top of page. “A learned man or talmud here is not clear. is forbidden to greet a menstruant. Rabbi Nechemyah says, ‘Even the utterance of her mouth is unclean.’ Said Rabbi Yochanan: ‘One is forbidden to walk after a menstruant and tread upon her footsteps, which are as unclean as a corpse; so is the dust upon which the menstruant stepped unclean, and it is forbidden to derive any benefit from her work.’” Therefore Rachel said, “It would be proper for me to rise before my lord to kiss his hands, but the way of women is upon me, and I cannot come near you nor walk at all in the tent so that you should not tread upon the dust of my feet.” And Laban kept silent and did not answer her, as it was customary not to converse with them at all because the speech of a menstruant was unclean.

ותאמר...כי דרך נשים לי, she referred to menstruation. The Torah elsewhere described this as אורח הנשים, when describing Sarah as no longer experiencing these cycles (Genesis 18,11).