וַיָּבֹא אֶל הָגָר וַתַּהַר וַתֵּרֶא וגו' וַתֵּקַל גְּבִרְתָּהּ בְּעֵינֶיהָ (טז, ד)
And he came to Hagar and she conceived and she saw that became pregnant… and her mistress became unimportant in her eyes. (16:4)
Sefer Sifsei Tzadik quotes his grandfather, the Sefas Emes zy”a, as explaining this pasuk as follows:
Lavan told Yaakov (Bereishis 31:29): “I have the power to inflict harm upon you, but the G-d of your father spoke to me last night, saying, ‘Beware of speaking with Yaakov either good or bad.’” Rav Bunim of Peshischa zy”a asks why Lavan told Yaakov that Hashem did not permit him to harm him. He answers that Lavan only merited prophecy one time, and he couldn’t hold himself back from bragging about it.
The Imrei Emes says that a similar thing can be said about Hagar. The first time she saw angels, she felt very proud. She thought that they had come for her, as a sign that she had reached a lofty spiritual level. Therefore, her mistress, Sarah, became unimportant in her eyes.
For this reason, the malach called her “Hagar, Sarah’s maidservant.” He was telling her that she only reached whatever levels she reached in the merit of being part of Sarah’s household. She then realized the truth and said that she was fleeing from “Sarah, my mistress”. In this way, she corrected her mistake.