The former Made In Chelsea star, 31, and her husband Hugo Taylor welcomed their daughter Sienna earlier this year.
Alongside a photo of herself with the infant, Mackintosh said: “Let’s talk about the baby blues.”
She said that after Sienna was born, she was both “totally overwhelmed by the love I felt for her” and “confused by the conflict in my emotions”.
“I’d go from being euphorically happy, to being deeply sad and tearful in the same moment,” she said.
“Hugo would ask what was wrong and I was unable to give him an answer which only made me feel worse as I couldn’t explain these all-consuming emotions.”
Mackintosh added that she felt “so stormy inside, which spiralled into anxiety as I feared developing post-natal depression (which did not happen)”.
She said she began to keep a diary which detailed how she was feeling and found that six weeks after childbirth she felt her mental state had improved.
Getting outside for daily walks also helped, she said.
She urged other mothers to “remember you are never alone”.
“If you are experiencing the baby blues don’t be afraid to vocalise it and if you feel like post-natal depression could be setting in then seek professional help as soon as you have those thoughts and feelings,” she said.