The answer: I broke myself a little bit. I started feeling fleeting twinges in my lower back at the start of the week and ignored them, blithely assuming they would soon disappear. But by midweek I'm in agony, unable to sit at my desk at work or walk or bend over without sharp bursts of intense pain. I head to a physiotherapist who diagnoses me with a strained disc - an unusual injury for someone fairly fit and not overweight, she says.
The Bikini Fit coaches are stumped, telling me that in years of running the programme, no one has ever injured their back as I have. And I can't recall a particular move or exercise that caused me any pain, especially under the watchful eyes of the trainers checking and correcting my form and posture. So what happened?
Watch: Sofia's week 4 Bikini Fit testimony
My physio says she believes the pain is a cumulative effect from months of workouts where I've pushed myself too hard, combined with a desk job that has kept my back muscles weak. Before starting Bikini Fit I would work out up to six days a week at the gym using machines and weights, with no supervision other than iPhone apps telling me I was "doing great!" And after losing about 12 pounds (5.4kg) in six months, I wasn't exactly going to argue with them, right?
But it looks like my gung-ho, "no pain no gain, bro!" attitude has backfired. As I feebly type this with one finger on my iPad I am lying in the foetal position with a pack of frozen peas pressed to my back, while strapped into a highly unattractive back brace that seems to have doubled up as a corset - the silver lining being that I quickly get too full before I can get too immersed in self-pitying chocolate binges. Yay.