Jessica Fostekew felt nervous yesterday, very nervous. So she went to lift some weights. “I was at peak dread,” she says. “So I was like, ‘Just do 20 minutes.’ Then I felt like doing another 20 minutes. That was the beginning of the reset. By the time I was on stage, I felt calm.”

Weightlifting has not only given Fostekew a way to boost her mood. It also inspired the standup show, now on tour, that has been causing all her nervousness. It’s called Hench, a term meaning strong and muscular. When a guy at the gym called Fostekew this one day when he saw her lifting, it threw her into a crisis.

On stage, dressed in gym gear, she explores this crisis, delving into her conflicted feelings about how weightlifting has changed her body. Comments from people around her (a “yuck” from her mum at the thought of women with muscly arms; her female personal trainer calling an athletic woman “unfeminine”) led to thoughts about the restrictions surrounding how her gender is allowed to be strong. It’s an energetic, joyful exploration of femininity and body image that won Fostekew rave reviews last year, as well as a nomination for the coveted Edinburgh comedy award.

Big things followed: Live at the Apollo, Harry Hill’s Clubnite, and her first TV panel show. But last night, the first date of the Hench tour, felt far more daunting. “I’ve never been cool enough to do any show here before,” she says, gesturing around Soho theatre. “It feels like the beginning of something massive.”

Fostekew, 36, started comedy in 2008. At the time, her ambition was to be an actor. But, a few gigs in, she “fell head over heels for standup”. However, her natural inclination to combine her acting talent with comedy was quickly crushed as she was told by a comedy promoter: “Don’t be too much like an actor.” That, she adds, came with loads of other messages: “Don’t talk about periods. Don’t talk about smear tests. Don’t dress too nicely. Don’t dress too scruffily. Don’t be too loud. Don’t be too aggressive.”

Fortunately for audiences, Fostekew has started to “unpick some of the bullshit tips I was given”. Her shows are now elevated by her astutely observed, brilliantly acted impressions of people in her life. In Hench, we meet PE teacher Dave, Fostekew’s parents, and her “sexist son”, who insists he could do his mum’s job better than her and pronounces Weetabix “pick-a-bitch”.

But her favourite part is animating her own “lowest, angriest” moments, with howling, flailing drama. The story of giving birth, where she realises physical strength has its limits as hypnobirthing fails her spectacularly, is a show highlight. “There’s a joyful catharsis in knowing how fun it’s going to be to do the very upsetting bits,” she says.

Hench takes an equally unflinching look at vicious cycles of dieting, the way food is linked to morality, and the judgments people make about “unfeminine” bodies. On stage, Fostekew wishes she could have heard this kind of thing when she was 11 and living on an apple a day. But changing minds is not the goal: “I’m grateful for people who’ve heard something that’s helped them – and, yep, I did that on purpose. But, equally, I’m happy for people just to come and laugh. I’m not denying the influence I can have, but I don’t think that’s my job. I’m a comedian.”

Fostekew’s podcast, Hoovering, takes a stronger stance. In each episode, she talks to a guest about eating. “Everybody’s history is so complicated,” she says. “They talk in such beautiful detail about their disordered eating.” To kick off 2020, Fostekew is offering her own alternative to weight-loss marketing: anti-diet industry episodes, with tips from experts about turning food into a much more positive experience.

She’s now working on a related TV comedy script set in her home county, Dorset, about a tight-knit group of female friends, their issues with eating and how these are passed down through families. Does she have a role for herself? “Myself 10 years ago – still in a cycle of thinking, ‘I’ll be happy one day when I’m thin.’”

Fostekew talks about the importance of female group dynamics, something she is keen to bring to the screen: “There have always been people saying there’s not enough stuff written about female friendship. Even in the mighty Fleabag, the best friend’s dead!”

Within comedy, too, Fostekew is part of a strong network of women. She’s a regular co-host on the Guilty Feminist podcast (the first seed of Hench began as an “I’m a feminist, but …”) and is full of praise for the support that is out there. “Doing standup made me a feminist,” she says. “If you’re a woman starting comedy now, you would find the most incredible sisterhood. Look at how many women are smashing it. There is no excuse for any lineup not to include a 50:50 bill now.”

In Edinburgh last year, she won the Comedy poster award with a striking photo of herself. Shot and conceived by Idil Sukan and designed by Chris Lince, the poster shows Fostekew standing, nearly naked, covered in chalk, preparing to lift a barbell. Getting to the final image wasn’t easy. “I hate having my photo taken. I did a lot of work in therapy to prepare.” But Sukan’s idea of a woman chalking up her entire body, preparing to take on any challenge, felt right. “I found the concept so moving. It’s someone who’s looked at the whole world and gone, ‘That looks heavy.’”

As Fostekew prepares for five months on the road, she knows weightlifting will bring those essential moments of joy that keep her going. “I’ve found a type of moving that makes me happy, that I always feel like doing, and I always feel better after. But it will always be for fun. I don’t ever want to be associated with #fitspiration!”

• Hench is at Soho theatre, London, until 25 January.

• This article was amended on 14 January 2020 to clarify that the image of Fostekew used on last year’s award-winning Edinburgh poster was shot and conceived by Idil Sukan; the poster was designed by Chris Lince.