I have been fainting at work over the last 4 months, and despite all manner of tests on my heart, circulation, nerves and blood the doctors don't know why. Somebody suggested postural hypotension, which means you get up too quickly from sitting and your blood can't circulate quickly enough. Has this happened to anybody else?
are you under stress, dieting, getting enough salt and liquid, taking any new medication, pregnant, newly menstruating? If its postural, the medics should be checking for this by doing lying and standing blood pressure checks. If your GP has checked all these things to no avail then ask to sea specialist.
I know someone who had this problem quite a lot as a teenager when he was growing rapidly (he's very tall anyway) but I doubt if it is as simple as that in an adult case.
Could it be low blood sugar? I suffer from it and have to avoid sugary foods and make sure I eat regularly as I go all shaky and feel faint? Otherwise it could be something as simple as anaemia, have you been tested?
I have naturally low blood pressure, and for me it is normal to feel faint every time I get up from sitting or crouching down. It's been like that all my life. However, if this is a new thing for you I would suggest you get the NHS to loan you a portable BP monitor so you can keep a regular check on it.
Don't know if this will help but this happened to me as a teenager. Had all sorts of test at Westminster Hospital, ECGs and EEGs etc. They put it down to a lack of glucose in my body and I had to build up my blood sugar eating dextrose glucose sweets.