The Brazilian was set to defend her title in a rematch with number one contender Valentina Shevchenko on Saturday (8 July).
However, just a couple of hours before the pay-per-view was set to commence, news emerged that Nunes was not feeling well and would have to pull out of the fight.
UFC president Dana White revealed that she had gone to the hospital where doctors medically cleared her to fight but "The Lioness" still had her mind set on not competing.
"Leading up to [Friday's] ceremonial weigh-ins, I got a call that she wasn't feeling well, and it was questionable whether she was going to show up to the ceremonials," White said, as quoted on MMAFighting. "Because the doctor needed to see her and they needed to find out what was going on with her, so, she does them, everything's good."
"This morning [8 July], I wake up and I hear that she's not feeling well again. And ... she's not feeling well again and she's probably not going to fight. So, I asked the doctors what's wrong with her. She was medically cleared. She was physically OK, they found nothing wrong with her, but she didn't feel right."
White then threw Nunes "under the bus" by putting her at fault.
"It's not like she was like 'I'm absolutely refusing to fight,'" White added. "She said 'I don't feel right, I don't feel good.' I think that it was 90 percent mental and maybe 10 percent physical, I think a lot of fighters have had times where they don't feel right."
"These situations arise all the time. I don't think we've ever had a situation like today, though, where she was physically capable of fighting. I won't do that [make her main event] again. I won't headline with that title again."
Nunes however, has since taken to social media to reveal her side of the story, explaining that despite making weight, she was unable to breathe and felt off balance from the pressure in her sinus.
"I have chronic sinusitis," Nunes posted on Twitter on Sunday (9 July). "I have fought with it before but this time it didn't work out, during the weight cut I was unable to breathe and felt off balance from the pressure in my sinuses.
"I was not feeling well enough to risk getting punched in the head with such pressure. I was taken to the hospital after weigh-ins and they only checked my blood and dehydration so cleared me based on that. The next day I went back and they did a CT scan and found a buildup and I was prescribed antibiotics and referred to a specialist.
"I have never pulled out of a fight before. I am sorry to all my fans that came out to see me, I'll make it up to you the next time I step in the cage. The fight is being rescheduled. I still want this fight. I still want this opponent."
While there is no confirmation on where or when the rematch could happen, it could take place at UFC 215 in Edmonton, Alberta, Canada on 9 September.