welcome to HCM!
After reading the query I can say form my clinical experience that the symptoms suggest that (most possibly) you have orthostatic hypotension- in which their is a sudden fall in blood pressure when one stands after sitting for some time.
I will advice that you should get your blood pressure checked- in sitting and lying down position and see if there is fall of blood pressure ?
also, you might be having some high blood pressure
which is not treated or treated partially and you need to revise the medicine.
You should also get thyroid
levels, sugar levels done, an ECG will also be helpful in ruling out other cardia cause.
You seem to be otherwise healthy person as nothing more is mentioned, so, the above causes are most possibly involved. I think by getting the test will pin point the diagnosis and solve the problem.
Hope the reply is useful
Feel free to ask more questions
Dr. Manisha Gopal