The singer, who fainted during a performance on the daytime chat show, said her pregnancy caused her to collapse, although she didn't know it at the time.
Speaking on Loose Women Rebecca said the unexplained fall caused her to take a pregnancy test, although it didn't initially solve the mystery.
"What's funny is after the show, I took a test because I was thinking ‘This is weird! What's going on with my body?'" she told the panel. "But it came up negative.
"I was like 'How strange! I've got all of the pregnancy symptoms,' but after the show I ended up spending two days resting in bed."
Rebecca explained that as the signs she was with child persisted she decided to get everything checked out.
"I kept on saying to everyone, I've got pregnancy symptoms so it was really weird," she added. "And then a couple of days later I took another test and it was positive!"
Rebecca Ferguson ollapsed on Loose Women
Liverpool-based Rebecca revealed two weeks ago that she is expecting her third child.
The 27-year-old, who is already mum to Lillie May and Karl, tweeted: "Yes I am having a new lovely baby children are such a blessing and I'm grateful to god for my new little gift xx."
Rumours had been swirling for several weeks after Rebecca's incident on Loose Women.
Rebecca Ferguson worried viewers when she fell to the ground
The singer, who recently released her second studio album Freedom, had just finished singing her single All That I’ve Got when she fell to her knees prompting co-host Myleene Klass to rush to her side and put her in the recovery position.
At the time the songstress had tweeted that she was feeling unwell.
"Still laid up in bed, dunno what this bug is but it's getting shaken off big tomorrow xxx," she wrote on Twitter.