But officers discovered Yang was healthy and did not display any symptoms indicative of the highly contagious novel coronavirus, which has so far killed 132 people and infected nearly 6,000 in mainland China.
They later learned that Cao had lied about her boyfriend's conditions, using the police to track him down after he dumped her following an argument.
Yibin police said Cao was detained for the "serious" offence of wasting police resources at a time of high alert over the deadly outbreak.
The authorities did not say whether Cao would be fined, or how long she would remain incarcerated.
Cao told investigators: "We argued, and I was very emotional. He texted me to say he's leaving, and that he wouldn't talk to me ever again.
"He then blocked me and I couldn't reach him.
"We'd gone very far in our relationship. We were close to getting married.
"I didn't want to let him go, but I couldn't find him through the people around him, so I decided to use this method."
She added however that she now realised she had been in the wrong saying: "I understand my way was too extreme. It affected public safety."