Others might not understand Su Chen, but she did!
Su Chen had only suddenly come to his senses a few days ago, completely transforming himself, but before that, he had been utterly defeated and self-abandoned. Back then, Su Chen wouldn't even bother to look at other girls, let alone her, Lin Lanxin.
As for the recent days, even if Su Chen were capable, it would have been impossible for him to win over someone with Xue Liluo's personality. Moreover, even if Su Chen had really caught up with Xue Liluo in just a few days, it surely would not follow that she was pregnant with Su Chen's child within those same few days, right? It's known that pregnancy usually takes about a month to be detected.
Therefore, Lin Lanxin was very certain that Xue Liluo was definitely not carrying Su Chen's child.