The former Lizzie McGuire star posted a photo of herself looking tanned and toned while on holiday in Cabo earlier this month.
Speaking on The Ellen DeGeneres Show, the green-haired star said she often has a "love/hate relationship" with her body but she was feeling good when she took that image.
"I've never done anything like that before," she said. "My Instagram page is pretty tame!
"I'd spent all weekend in a bikini and I was feeling pretty good. I'm just like everybody else, usually I'm not feeling so good and I have a love/hate relationship with my body."
Hilary said she'd been on holiday with son Luca, three, and that's what prompted the photo.
"It's been three years since I had him and I'm so proud of my body and what it's done for me. It gave me the most beautiful little boy and I felt strong and powerful and wanted to inspire other women."
Hilary is currently promoting new TV show Younger, which starts in the US tonight.
It's a new comedy series from Sex and the City creator Darren Star about a 40-something woman who finds she's been shut out of the job market after taking a career break, so undergoes a makeover and pretends to be 26 to get back in.