וַיָּבֹ֥א אֶל־הָגָ֖ר וַתַּ֑הַר וַתֵּ֙רֶא֙ כִּ֣י הָרָ֔תָה וַתֵּקַ֥ל גְּבִרְתָּ֖הּ בְּעֵינֶֽיהָ׃
And he went in unto Hagar, and she conceived; and when she saw that she had conceived, her mistress was despised in her eyes.
Rashi on Genesis
ויבא אל הגר ותהר AND HE CAME UNTO HAGAR AND SHE CONCEIVED from the first union (Genesis Rabbah 45:4).
Radak on Genesis
ויבא ... ותקל גבירתה, she thought that now that it was clear that Avram's seed would be from her she would become the top ranking wife of Avram. As a result, she refused to carry out instructions given to her by Sarai.