You have vaso-vagal syncope which causes a drop in blood pressure, light-headedness, and abdominal discomfort (gas, red or pale yellow face, and urgency).
Vaso-vagal syncope is caused by a reflex which usually occurs while standing or after eating. The reflex triggers an increase in heart rate, force of contraction, and resistance of peripheral vessels. When you stand up blood pools in your legs which results in your heart becoming less full which triggers a contraction of your heart which causes blood vessels to dilate and your blood pressure to drop. When your blood pressure drops then your brain becomes inadequately perfused and you feel lightheaded.
If you are dehydrated then your blood volume is less than normal which causes it to beat faster and more strongly which triggers vaso-vagal syncope.
Blood pressure medications and alcohol can cause excessive drops in blood pressure and vaso-vagal syncope.
After a large meal the blood vessels in your GI tract dilate so that digestion can occur and blood pools in your abdomen which decreases the amount of blood that returns to fill your heart. So that if you stand up then blood will pool in your legs. This is called post-prandial hypotension.
Also lack of sleep can lower blood pressure when upright so that you will feel lightheaded when standing or develop vaso-vagal syncope.
Medications called beta blockers are used to treat vaso-vagal syncope.
Increasing the amount of salt and water in the diet can raise blood volume. A low salt diet can be harmful for someone who has low blood volume with recurrent light-headedness.
Florinef is a medication that helps the body retain salt and water and expand blood volume.
Stand with your legs crossed and squeeze one leg into the other to activate the muscle pump in your legs to move blood from the legs into the chest.
A doctor can diagnose your vaso-vagal syncope using a tilt table: http://www.syncope.co.uk/ttt.htm