The soap star, 32, who plays Eva Price in the ITV soap, shared a series of videos of herself and personal trainer Matt Warner baking a chocolate cake - including one clip where a cardboard cut out of her at her heaviest appeared from behind a kitchen counter.
'Fat Cath tried to make a comeback', she quipped to her 124,000 followers.
Many were shocked to hear of Catherine's former weight struggles, as she's since shed an impressive six stone through an intense diet and exercise regime.
'I was amazed when I heard that this perfect lady had the same issues as some of us are facing. To see the way she looks now is a real inspiration, I have her Fat Cath stuck in my mood board for when I need a pick up!' one such fan wrote.
Speaking in an interview with the Mirror, Catherine, who welcomed her first child Alfie in March 2015, explained of her health overhaul, 'It has to be a lifestyle. Diets do not work. Fact. I have done every diet going.'
'I was treated differently. I was always the fat, funny friend.'
'I went back to the gym less than six weeks after giving birth. But safely. I was doing hill sprints with him in his pram. There’s a little bit more pressure with my job. Especially because Eva is always in ridiculously tight dresses. My dad always has to avert his eyes.'
She added that in addition to her workouts, she's completely changed her eating habits to achieve her goals.
'Tom [Pitfield, Catherine's now husband] and I have a rule. If it ran, swam or grew from the land, we eat it. We don’t eat processed food.'
'[For a treat] I’ll do a big gluten-free pizza with lactose-free cheese. Now that I eat so clean and avoid all junk, I just feel ill when I eat it. We make our own sweet potato chips too as a treat. I’m so rock ’n’ roll aren’t I?'