Guilty without a trial
When Luo Muchen stormed out he didn't notice Xiao Fen standing with her back pressed against the wall. She had seen everything that had transpired in the room through the paper window. When Luo Muchen left the study at first she thought he had run back to his new bedroom that he had been staying in ever since he got married but when she got there he wasn't there. She figured he might have come here on instinct but she didn't expect Qie Xieling to take advantage of the opportunity she created. Filled with anger and irritation she waited for him to leave before slipping into the shadows. She had to do something fast before the truth of her drugging Luo Muchen was revealed. This was the future clan leader and drugging them was a sin that couldn't be forgiven.
When Xiao Fen meant she would do something about it she meant that she was going to subtly spread a rumour that Qie Xieling did it.