The former Spice Girl, 39, was absent from the ITV show on Saturday night but was back yesterday after being released from hospital.
Some fans said she appeared to have scratches on her body and a bruised cheek.
Her husband, film producer Stephen Belafonte, denied the “disgusting” speculation in a message on Twitter.
“I don’t usually respond to Twitter messages but I will respond to comments of hitting my wife which I think are quite disgusting... untrue!” he wrote.
“Mel was very ill. A bunch of doctors helped her get better. If fans can’t just relax before being negative they are not real fans of Mel B.”
The singer, who is from Leeds, did not appear to be wearing her wedding ring when she returned to the second part of The X Factor results show yesterday evening, when Ben Haenow was crowned winner.
Tulisa Contoslavlos replaced Mel on Saturday’s show.
On her return on Sunday, Mel thanked the medical staff who cared for her during the illness.
She said: “I really missed being here but thanks to all the doctors and nurses who took care of me and got me here tonight.”
Asked by host Dermot O’Leary if she was feeling better, she replied: “Yes - ish.”
Her appearance overshadowed the victory for van driver Haenow, 29, who beat hot favourite Fleur East, 26, on the show.
Haenow said afterwards: “I don’t believe it. I don’t believe it. Thank you so much.
“My dream has come true. You can’t imagine you are going to get to this point in the competition.”
He tweeted: “Cannot explain how amazing this feels and how grateful I am to everyone who voted! Happiest man alive right now Xx
“I’ve actually got a SINGLE out!!!!!!!!!!!!!!!!!!!!!!!!!”
Simon Cowell, who was mentor to both of the final two acts, hailed East as a “fantastic runner-up” and said: “I couldn’t be happier for you, Ben. You’ve deserved it - you are a great guy and I’m so proud of you.”
Official figures released after the show revealed that Haenow secured more than half of the public vote in the final, with 57.2%. East polled 34.3% with third-placed Andrea Faustini getting the remaining 8.5%.
The data showed that Haenow was the most popular act in every vote after the third week. In the first three shows Faustini received the most votes.
East, who was the bookmakers’ favourite before the weekend, was not the most popular contestant in any of the votes.
She was gracious in defeat, saying: “I’m so grateful. I love Ben so much, I’m so happy for him.”