The singer, 37, recalled feeling “crazy fat” and “ugly”, which she said was made worse after she fell pregnant with her now 17-year-old daughter, Tahlia.
“I just didn’t feel good. On top of that having baby blues with the girls, it didn’t make me feel good. I felt like I had this baby belly. I felt crazy fat,” Buena told the BBC Sounds When I was 25 podcast.
She said this was made worse by having to return to work so soon after giving birth.
“I remember feeling really, really low,” Buena said. “I didn’t’ feel sexy, I didn’t feel like my normal self.
“No woman feels themselves after having a kid it takes a while to pull yourself together, but I didn’t have that.”
Tahlia was born in March 2005, seven months before the band released their fourth studio album, Taller in More Ways in October that year.
Two months later, Buena left the group because of “personal reasons”. She later disclosed that she left because she was suffering from postnatal depression.
The singer spoke candidly about trying to juggle motherhood and being in a successful pop group while trying to hide her depression, which she said felt too taboo to talk about at the time.
“I didn’t realise for two years. For two years I had no idea I was even suffering with depression,” she said, explaining that she just felt “really emotional”.
“Suffering with depression was the hardest because you have to then own up to what you’re going through.
“For ages, it felt embarrassing. You don’t tell people you have depression because then they look at you different. Talking about it makes you look crazy.”
She said that she would often sit in the dark with her curtains closed all day long, before deciding that she “needed to do better for [her] daughter and for [herself]”.
“Even now I still have that feeling of not being the best I can be or feeling like I’m not good enough. But I’ve learnt how to pull myself out,” Buena added.
In June, the original Sugababes members – Siobhan Donaghy, Buena and Keisha Buchanan – announced a new 17-date tour later this autumn. It marks the band’s first UK tour in more than two decades.