My mother found out after a year suffering what you are from, that she was diabetic. Her sugar level would go very high to very low of course depending on what she ate or drank. You need to get a blood test done pretty fast because it is manageable. Now she is type 1 and my Father is type 2 and even though its not funny we would laugh cause he would almost go right to sleep after eating anything sweet. Then he found out he was type 2 and doing good on the medications from his doctor. Get it checked out now do not wait.
It could be that you have post-prandial hypotension, which means that your blood pressure drops very low after eating. This can cause excessive sleepiness after a meal. Do you have any other reactions, such as dizziness or light-headedness, or even anxiety?
It could also be from hypoglycemia (low blood sugar).