Why did Hagar despise Sarai?

Answer From The Annual Reading Schedule - Week 4 Notes For Genesis: It is not really clear to me why Hagar despised Sarai when Hagar conceived and was pregnant. I think it is possible she despised her knowing that the child would not be hers, but Sarai's, since she was Sarai's maidservant. However, I am not certain if this is the answer.