Other symptoms, beside lightheadedness?
If it was FOOD ALLERGY (probably to fish, shellfish, nuts or tree-nuts), you'd probably also notice throat or skin itching, lips or face swelling, transient rash, or diarrhea. If lightheadedness occur only after particular food, you should have a skin test for allergy.
If it was DUMPING SYNDROME (rapid stomach emptying), you'd probably also feel hunger, irritation, tremor, profound weakness, anxiousness (due to hypoglicemia followed by adrenalin release). This would happen especially after SUGARY meals. It is the gastric emptying test, which can show this.
POSTPRANDIAL (meaning after-eating) HYPOGLICEMIA may be due to developing diabetes. It is an after-meal blood glucose test, which can confirm this.