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.