Another interpretation: "sweetness" [nofet] it was not mine the words of Torah of which it is said (Psalms 19:11) "the drippings of the honeycomb" [nofet tzufim] were destined to be spoken in the portion of Naphtali. (Genesis 30:10) "And Zilpah bore." Of all the others it is written "and she conceived," but here "and she bore" rather, she was a young woman and her pregnancy was not noticeable.
Naphtali the Sweetness of Torah and Zilpah's Hidden Birth
Yalkut Shimoni on Torah 127:4
דָּבָר אַחֵר, נֹפֶת, לֹא שֶׁלִּי הָיְתָה דִּבְרֵי תּוֹרָה שֶׁנֶּאֱמַר בָּהֶם (תהלים יט, יא) "נֹפֶת צוּפִים" הָיוּ נֶאֱמָרִין בְּחֶלְקוֹ שֶׁל נַפְתָּלִי". (בראשית ל י) וַתֵּלֶד זִלְפָּה. בְּכֻלָּן כְּתִיב וַתַּהַר וְכָאן וַתֵּלֶד אֶלָּא בַּחוּרָה הָיְתָה וְלֹא הָיְתָה נִכֶּרֶת בְּעִבּוּרָהּ.