Thanks for your health concerns. When you are sitting or laying down, venous return to the heart is easier; once you stand up, a big portion of the blood pools in your legs and your body maintains perfusion to the brain by increasing heart rate
and blood pressure, both which you may feel, so it may be an appropiate neural reflex.
As for the phlegm, there are many causes of chronic phlegm, including allergies, COPD
, hay fever
, etc. If you have a history of smoking
it is most likely linked to that.
Hope that is useful,
Dr Brenes-Salazar MD
Mayo Clinic MN