I'm much better at it now than I was years ago. I still have the problem with people being pushy with me and trying to talk me into things, and I end up saying I'll think about it. And then I might just flake on them.
Seems to me that when employees are calling in sick, they leave a message very early before the work day starts, then probably wouldn't answer their phone if I called them. Sometimes I might text them if I have an important question about something, but I'd be apologizing for bothering them while they're ill. Luckily no one's chronically absent, but people get sick and have to miss sometimes.
If it happens again where you're ill and have to stay home, think of it as you are notifying your boss, not asking for permission (not to say that, just approach it that way). And the best response to the guilt trip is probably, "I'm so sorry, but I need to stay near the bathroom today." How is she to argue with that?
Feel better