She made her comments on Twitter after one user criticised her and fellow working mother Holly Willoughby.
The Coronation Street star has three children - three-month-old Polly with Hollyoaks star Jamie Lomas and Emily, 13, and David 16, from a previous relationship.
A user tweeted: "(Don't) know why celebs like @msm4rsh and @hollywills leave their babies for tv it's ridiculous."
Kym retorted: "It's called working... or should we not be working when we have children?? How dare we provide for our kids! Idiot."
She added: "Actually I DO need to work and I am NOT married.
"I have bills just like everyone else, no work no income.
"And my child was with my mother not a nanny not that it's your business!"
The exchange on the social networking site came after the 35-year-old former pop star revealed she was filming an appearance on David Walliams' TV show Wall Of Fame.
Kym said: "There are people who can't go to work and I have been on that side too but I won't be criticised cos I do.
"I have lived on benefits too cos I couldn't get work that covered the cost of living with two small kids so I sympathise.
"I cannot believe that in this day and age people still think mums should not work!
"Claiming I don't NEED to work? What am I? Made of money?
"I suppose I should stay home until I am broke again then should I?"
She added: "Also why is it ok for dads to go out to work then? But not mums??? Sexism???"