Thank you for your query at DoctorSpring.com.
I understand your concern.
You could be having a sudden lowering of blood pressure on change of posture from sitting to standing up. This is termed as postural hypotension. When the blood pressure decreases on standing, you get lightheaded and experience the other described symptoms. This is nothing to worry about as it is quite common in the elderly due to impairment in autonomic reflex (the mechanisms in our body which regulate our blood pressure etc) as a result of aging.
High blood sugar or diabetes also contributes to the autonomic dysfunction and makes you more susceptible to postural hypotension. In addition dehydration can also worsen it.
Normally on assuming a standing posture, blood pools in the legs, resulting in less blood reaching the heart and a mild lowering of blood pressure which is not symptomatic as a normal autonomic reflex compensates and prevents a large fall in BP.
However, when autonomic reflexes are impaired, the fall in blood pressure is progressive and more, after standing because the gravitational pooling of blood in the legs cannot be compensated.
I suggest you drink plenty of water and remain well hydrated. This will improve your blood volume and reduce chances of a postural fall in BP.
In case your symptoms persist, I suggest you consult your GP for further workup.
I hope this was helpful. Feel free to ask queries.