I don't know if you have to tell her, but it makes life easier if you know what might be going on (maybe I'm just a nosey boss!). Can you say something to keep her quiet, without saying exactly whats going on?
e.g. when I had an ectopic pregnancy removed, the sick note just said surgery.
Hi I've recently been having a few medical problems and have had a few doctor/hospital appointments during working hours. My manager has been fine about it and give me time off to go. I have another appointment on Thursday which will require some time off work. I asked my manager if I could start later on Thursday which she was fine about.
I have not told my manager the nature of my illness but I feel it will only be a matter of time before she asks. The thing is I don't want to tell her what's wrong, I'm worried that she will tell other staff as she is quite friendly outside work with a few staff and I'm worried she will get drunk and tell them.