An undisclosed medical condition has forced unified women's middleweight world titleholder Christina Hammer to postpone her fight with fellow two-belt titlist Claressa Shields for the undisputed championship.

Shields and Hammer, who have been on a collision course for the past year, were scheduled to fight Nov. 17 in the main event of a Showtime-televised card at the Adrian Phillips Theater at Boardwalk Hall in Atlantic City, New Jersey.

But Monday, Shields wrote on social media that the fight was off for now and she was not pleased about it.

"Very, very, very disappointed. I'll let my team do the announcement! But what I will say is I want this fight 100%," she wrote. "I am not the one sick. Idk (I don't know) the ins and outs of Hammer's condition, but she can't box til March the doctor says. I'm still going to fight the 17th of Nov. I really just need an entire day of sleep to make me feel better. I'm so damn mad right now. Like I wanna punch a hole thru a brick wall."

Whether Shields fights somebody else Nov. 17 remains to be seen.

"I told Claressa that I will have all the discussions necessary with all the relevant parties to try meet her desire to fight in 2018, but those discussions haven't taken place yet," Mark Taffet, Shields' manager, told ESPN.

Taffet said they were recently informed that Hammer had some kind of medical issue that would keep her from the fight that was formally announced just last week.

"We were informed it is not an injury but a medical condition for which she will be treated and expects to fully recover," Taffet said. "We are just beginning discussions with Hammer's team, [Shields' promoter] Salita Promotions, Showtime and Boardwalk Hall so we have nothing further to report at this time."

Shields added that she would "stay focused and in the gym."

The winner of the fight, should it be rescheduled, would become just the second woman of the four-belt era to hold all the major world titles in one division. The other is reigning undisputed women's welterweight world champion Cecilia Braekhus (34-0, 9 KOs), the No. 1 women's pound-for-pound fighter in the world.

Shields (6-0, 2 KOs), 23, of Flint, Michigan, who won back-to-back Olympic gold medals in 2012 and 2016, claimed two super middleweight world titles in her fourth fight 13 months ago. After one defense, Shields dropped down in weight for a June 22 fight against Hanna Gabriels for two vacant middleweight world titles. Although Shields suffered the first knockdown of her career in the first round, she rallied for a clear unanimous decision.

Hammer, who has held a middleweight world title since 2010, cruised to a shutout decision against Tori Nelson in a title defense on the Shields-Gabriels undercard that was intended to set up their eventual undisputed title showdown. Hammer (23-0, 10 KOs), 28, of Germany, who has also won a world title at super middleweight, has made 12 title defenses.