The baby indeed didn't belong to Huo Zihang.
However, it also existed because of him.
On that day, she was humiliated by her peers who mocked her by saying that Huo Zihang definitely wouldn't marry her. Hence, she thought that she should climb onto his plate.
She pulled some connections and managed to contact Huo Zihang's friends, and then asked them out to the bar for some fun. She did her best to get Huo Zihang drunk and she was initially going to succeed!
When she was reminded of the fact that she got shoved into the elevator by a drunkard and forcefully taken to the room beside Huo Zihang's where he violated her, a great sense of humiliation suddenly arose.
In order to marry Huo Zihang, she had already paid such a big price, so she must succeed!