The singer, 34, insisted she feels "great" after struggling with her body confidence due to past pressure in Hollywood to look "perfect".
As reported by The Sun, during a performance at KOKO in London she told the crowd: "I have gained weight and I've not been to the gym in ages, but honestly, I feel great.
"I'm not 17 anymore and I want to make a baby and I am trying to do that and my body is changing shape."
The star, who first appeared on stage in the West End aged 11, rose to fame after the release of her single 'Do It Like a Dude' in 2011.
Recalling the early years of her career, Jessie said: "I was in LA by myself and I remember looking in the mirror and being obsessed with being perfect. I felt like I had to be who people wanted to be accepted.
“I put on a bikini and dry humped a horse in 'Price Tag'. It was weird. I just felt like I lost myself in Hollywood."
She added: "This whole celebrity thing is so weird to me and how people are even more obsessed with how I look than I am."
It comes after Jessie revealed on Instagram in November last year that she had experienced a miscarriage after trying for a baby "alone".
The singer revealed that she was "heartbroken" after being told there was no heartbeat at her third scan.
She added: "But I will keep climbing out of the sadness, my happiness is my responsibility not anyone else's.
"I will not be putting bandaids on the problems and heavy feelings but really feeling them and healing them. Gotta practice what I tell everyone else to do in these damn songs."
Watch: Jessie J hopes to start family with partner after going through miscarriage alone