Sallust fills us in on that.

One of Catiline's conspirators was a dude named Quintus Curius. He had been fooling around with Fulvia. She was an expensive girlfriend, and Curius ran out of cash. To keep her around he started bragging about how he was going to become wealthy and she would have everything (after Catiline overthrew the government), and also threatened to kill her if she said anything.

She told several people what was up, and Sallust says this rumor was one reason Cicero won election to consul that year.

Once Cicero became consul, he used Fulvia as a go-between to flip Curius. When Catiline made the plan to assassinate Cicero at the meeting in Laecus' house, he sent Fulvia to Cicero with the warning. She got there in time to foil the plot.