פירוש על בראשית 16:4

ויבא אל הגר ותהר AND HE CAME UNTO HAGAR AND SHE CONCEIVED from the first union (Genesis Rabbah 45:4).

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.