Peritonitis.
Mono.
The initial symptoms (in an adult) are fatigue and nausea and fever - precisely the sort of thing your suggested personality would ignore, or dismiss as “flu”.
Granted, it is almost impossible for it to be fatal in a developed country, but if the woman keeps working and ingoring her ilness, she might wind up with a very dramatic and prolonged hospitalization - someone I know just collapsed in the street, and woke up three days later in a hospital, with wires and tubes stuck all over, and no idea what had happened.
You could write so the doctors at first had noe idea what was wrong with her, and she herself convinced she is going to die - believe you me, mono makes you feel that way.
The Gramps from Hell reckons that a healthy diet involves mostly beans (f in soup version, the liquid must have a thumb-thick layer of fat floating on top), steak and shellfish. Heeeeey, dude was doing Atkins before Atkins! But I digress.
For one week, when he was in his 40s, he’d been feeling “gassy.” Beans didn’t sit well with him, heck, nothing did. Finally he decided to go to the hospital. He got on the tram, but the vibration made the pain worse, so he got off and walked the 3 miles or so to the hospital (uphill but no snow).
“Peritonitis explosiva:” it had been apendicitis, but by leaving it unattended, it had not just turned into peritonitis but ruptured. He spent a week in the hospital before they kicked him out. Most other people would have needed a couple of weeks to recover, but that man is made of cast iron except for the bowels.
In her early 30s, Mom had pain in her right groin, but it came and went, so her doctor didn’t take it seriously - between the time when it hurt and the time she got to his office, the pain would be gone. Finally she mentioned in front of one of Dad’s uncles, a doctor himself, who convinced her to get it checked by a different Doc, as it sounded like apendicitis to him except for one symptom. The other Doc said it did sound like apendicitis indeed, so “let’s open.” They open and her appendix is just fine and happy, so they start looking around and discover that she has a HUGE cyst in her right ovary, which couldn’t be felt from outside as it was on the inner part of the ovary. The ovary was somehow “twisted” out of place by the growth, too. They called an Gynecological surgeon and took out both the appendix (“since we’ve already opened and anyway it’s useless”) and the ovary. We know for sure my brothers are both from the left one.
My paternal Grandma had DVT in ther 70s and from what she said it didn’t hurt a lot. It did hurt, but compared with other pains she’d had it was a walk in the park. She was more scared from seeing the look in the doctor’s face than from the pain. The doctor determined that she was thrombosis-prone and she was on anticoagulants for the rest of her life.
Rockin’ pnuemonia, comorbid with Boogie-Woogie flu. Tragically common among today’s young people.
Some sort of heavy metal (oh, and she’s a DJ! Irony!) poisoning?
Varicella pneumonia. It’s a complication of chicken pox in adults. The mortality rate is 10-50% so it would be a very scary illness for someone who hated hospitals to begin with. Especially since she’d have to feel terribly unlucky to be amongst the 10% of people who get it without lymphoma or an autoimmune disorder.
Just ditto-ing Hodgkin’s Disease, as it’s a highly curable cancer. 95% remission rate, 85% cure rate (cure being 5 years of remission), and those numbers include all stages of it from I to IV. So if you have stage II, you pretty much know you’ll be fine as long as you go through six months of chemo. You just have to go through six months of chemo, which is no small thing. (And chemo also depresses the immune system, so there’s always a risk of dying from a secondary infection, like a cold. I wasn’t allowed to eat raw produce because of the risk of bacterial infection.)
It’s also more common in people aged 20-35. (And those aged 50+, but that doesn’t apply here.)
Just a fun little tidbit, since she’s in the club scene – one of the more uncommon symptoms of the disease (which can be nearly asymptomatic early on, but is usually indicated by night fevers and weight loss) is “pain in the affected lymph nodes after consumption of alcohol.” Basically what it feels like is the same kind of burning acheyness you get with a really bad case of the flu. I got it across my shoulders, upper back, and down the arms to about my elbows. It came nearly instantly after drinking any amount of alcohol, ranged in intensity from annoyingly noticeable to take-my-breath-away, and always faded within about 5 or so minutes. It was, btw, very easy to write off as “weird, but not worrisome,” so that could have some play in your story as well.