והשקה את האשה. אֵין זֶה סֵדֶר הַמַּעֲשֶׂה, שֶׁהֲרֵי בַתְּחִלָּהּ מַקְרִיב מִנְחָתָהּ, אֶלָּא הַכָּתוּב מְבַשֶּׂרְךָ שֶׁכְּשֶׁיַּשְׁקֶנָּה יָבֹאוּ בָהּ לְמָרִים; לְפִי שֶׁנֶּאֱמַר בֶּטֶן וְיָרֵךְ, מִנַּיִן לִשְׁאָר כָּל הַגּוּף? תַּ"ל ובאו בה — בְּכֻלָּהּ, אִ"כַּ מַה תַּ"ל בֶּטֶן וְיָרֵךְ? לְפִי שֶׁהֵן הִתְחִילוּ בַעֲבֵרָה תְּחִלָּה לְפִיכָךְ הִתְחִיל מֵהֶם הַפֻּרְעָנוּת (ספרי): והשקה את האשה AND HE SHALL CAUSE THE WOMAN TO DRINK [THE BITTER WATER] — This was not the actual order of the proceedings, because really he offered her meal-offering first (before giving her the water to drink; cf. v. 26, 27), but Scripture here merely informs you that when he afterwards gives her the water to drink it will become bitter in her belly (cf. Sotah 19a). — Since only the thigh and the belly are mentioned (vv. 21. 22), I might conclude that only these are affected (see Sifrei Bamidbar 18); whence do we know that this applies also to the rest of the body? Because Scripture states here, “[and the water … shall come] into her [and become bitter]” — into all of her body! But if so, why does Scripture explicitly mention only the thigh and the belly? Because they (these two) were the first to begin the sin, therefore it (Scripture) makes the punishment begin with them (and for this reason Scripture mentions them explicitly, but, as a matter of fact, it extends to the whole body as is implied in the word בה (Sifrei Bamidbar 18).
למרים. לִהְיוֹת לָהּ רָעִים וּמָרִים: למרים (The text is more lit., “and the waters shall enter into her as bitter”) — i.e. to become bad and bitter for her.