She apparently obtained a poisonous substance from her boyfriend, Shahid Lashari, last week and mixed it in milk for her husband, who refused to drink it, Tajak said.
The woman's mother-in-law later inadvertently used the tainted milk to make a traditional yogurt-based drink and served it to 27 members of her extended family, who fell unconscious and were hospitalised.
Seventeen people died and 10 are still being treated in hospital, he said.
Bibi and Lashari appeared before a judge in the city of Muzaffargarh on Tuesday, where she told reporters that she was angered over her parents' decision to marry her to a man against her will. They did not have lawyers.
"I repeatedly asked my parents not to marry me against my will as my religion, Islam, also allows me to choose the man of my choice for marriage but my parents rejected all of my pleas and they married me to a relative," she said.
She said her love affair with her boyfriend continued after she got married.
Bibi said she had warned her parents that she was capable of going to any length to get out of the marriage, but they refused to allow her to get a divorce.
She said Lashari gave her a poisonous substance, which she used to try to kill her husband. She expressed remorse over the deaths, saying her target was only her husband.
Tajak said police were trying to trace and arrest all those who were aware of the plot. He said Lashari confessed to supplying the poisonous substance.
Faisal Chingwani, a top human rights activist in the city of Multan, said Bibi apparently committed the crime because she was mentally stressed about the forced marriage.
Many parents in Pakistan arrange marriages for their daughters against their will.
Also, in the eastern city of Lahore, a brother shot and killed his sister who had recently wedded a man of her choice without consent from her family in the latest case of so-called honour killings.
Police officer Shaikh Hammad said Mahwish Arif, 25, was fatally shot by her younger brother Samar Ali. The brother fired three bullets as Arif came to visit her parental home, months after the marriage, in the suburban area of Satu Katla.
Hammad said Ali fled after killing his sister. A police search was under way.
Nearly 1000 Pakistani women are killed by close relatives each year in honour killings.
- AP