I think on balance you're better telling her than not - but in the way AMumInScotland describes, i.e. just as an FYI I wanted to let you know this is going on. That way if she is unsympathetic you haven't said too much and if she is sympathetic, you can say a little more if you want.
If your manager is usually pretty decent about illness etc then I'd suggest telling her, you only have to say "I've not been 100%, getting some tests, hopefully nothing serious but it's making me tired" etc. I manage staff and I'd rather they had a quiet word so I know they're not feeling quite right rather than struggling on and me maybe thinking they hate the job or are pissed off about something.
yes tell her so that she knows what's going on. she may need to arrange cover for you if you have to go off sick and she can help re distribute your workload if you are finding things too much. also, from an emergency POV, if you collapsed, it's better for the person ringing ambulance to know as much as possible about the patient. (dont want to scare you but it happens)
I've not been feeling well for the past two weeks, however I've not been sick off work. I have lower right abdominal pain which the doctor thinks may be ovarian cysts (im waiting for a scan)and I have been tested for ovarian cancer (have a family history of it). I'm feeling tired at work and finding it difficult to concentrate, I haven't told my manager I'm feeling unwell and now I'm worried that she might think I'm not capable of doing my job should I tell her or wait till I know what's wrong? I feel silly telling her when I might not have anything wrong with me