She took every opportunity to rub Sarah's nose in the fact that she had given Abraham what Sarah could not. So, there is a sense in which Hagar is reaping what she sowed too. We are confronted here with the painful truth that there are consequences to our sin. Hagar became pregnant. I suspect problems began from the very first night Abraham stayed in Hagar's tent.
Sin brings consequences.
Now, I believe God had forgiven Abraham. He had even promised to bless Ishmael when Sarah wanted them to leave once before. But, the negative consequences remain. This is a fact that we have to face. The person who engages in illicit sex may face consequences of disease, pregnancy, a broken relationship or the guilt of having given away something precious.