When describing symptoms, orthostatic intolerance and presyncope might be useful terms to use, although I realise that they only convey a small part of the whole experience of the illness. However, doctors are at least familiar with these terms.
Lately I have been thinking of mentioning executive dysfunction in regards to myself, although it is very bound up in particular diagnoses and I would suspect many doctors don't know that it can occur for all sorts of reasons in a range of illnesses and injuries.
At the start of my illness, 20 years ago, I kept saying that I felt as though I had low blood sugar. I wonder now whether this was counterproductive. It did get me some testing for blood sugar problems, including (eventually) for reactive hypoglycemia, and those were good tests to have, although they turned out normal. I wonder now whether it would have been better to say that I felt as though I was going to faint.
Of course, back in those days I had never heard of orthostatic intolerance, although I had experienced it occasionally and transiently, as most people have.
There's no point dwelling on it now, I suppose. I do think it is up to doctor's to ask good questions though, and tease out information about how people are feeling.
For instance, after reading online forums, I was surprised to find that many people use "dizziness" to describe feeling faint. Growing up in Australia, I only ever heard dizziness used in the sense of how you might feel after spinning around in the garden or going on a fun-fair ride. To me, dizziness means that there is some (unpleasant) sensation of spinning. (I don't know whether this is typical of Australian usage, or just of my household.)
I wonder how other languages describe these feelings, and how they divide them up.