Hi Eleanor

Welcome to Neurotalk.

It sounds like you may be experiencing Orthostatic Hypotension (aka Postural Hypotension). This is a sudden drop in blood pressure (BP) on standing that can cause dizziness etc. When it happens sporadically and on it's own it's usually not a cause for concern but persistent occurrence can be a symptom of an underlying illness.

It would be worth you getting your doctor to do a Postural BP test. Note if you are stressed during your check up it can be enough to raise your BP back to normal (or above) and hide the symptom - something as simple as bad traffic on the way to the clinic will do it - so try and stay calm before the test.

I hope you find some answers soon.