I don't think anyone here is downplaying the seriousness of the situation, but that the OP is of an age where she needs to learn to tackle these situations by herself. She is physically and mentally able of addressing the situation, so it's not appropriate for a parent to step in. It is emotionally difficult but it's something that everyone needs to learn..
Of course it is. But some of the posts here seem to be a little harsher than necessary in order to help this 23 year old learn that. She's still young and learning. We can guide her here and help her.
Also, we have to remember that the OP has a serious medical condition. That makes it much more difficult to "tackle these situations" or even learn to tackle them. It isn't so simple as being an adult who is and should be responsible for herself, who is both mentally and physically capable of addressing the situation. Yes, she needs to learn how to do this, but the difficulties are much more than just emotional when you have a serious, chronic medical condition.
I am speaking here as someone who has several chronic illnesses for many years and has had to deal with getting appropriate time off, etc. Sometimes I have navigated such problems more successfully than other times, I will admit, but you don't exactly get a handbook along with your diagnosis of disorder xyz about exactly how to solve the social problems it causes.
You just need to do the best you can, is what it comes down to for me. Did the OP's father overreact? I think if he had intervened strictly on the basis of getting time off, it might have been an overreaction. But he was reacting to the fact that his daughter almost died. I think any parent, regardless of the age of the offspring, would have a strong reaction to that even if they didn't do what the OP's dad did.
And sometimes, in such situations that involve serious chronic illnesses, you actually need the help of someone else, be it a parent, a spouse, a sibling, whatever, to help you handle work problems, etc. It's unfortunate but I have found it to be true-such as when I cannot physically call my workplace to tell them I am sick, even on the way to the ER, and I ask a relative to do it. If the father overreacted in this particular case is a matter of debate but as an overall point, I don't see anything wrong with getting help as needed from a family member, even if it wouldn't be the best thing to do if you were physically healthy overall.