It could be low blood sugar. You problem sounds a lot like mine. Unless you've checked it while you you feeling bad, it would be hard to tell. A 4 hour glucose tolerance test usually shows reactive hypoglycemia but it can be awful. Docs usually use the 4 hr GTT to check for diabetes so they will tell you the test is normal even if your blood sugar dropped really low during the test. Many people can go low without symptoms but if you go low or drop suddenly and have symptoms such as yours, you probably have reactive hypoglycemia.
You don't even need to diagnose it to treat it. Treatment is through diet. You need to eat something about every two hours from when you get up until you go to sleep. Every meal/snack should have some protein, fat, and complex carbs. Stay away from pasta, potatoes, bread, deserts, and fruit at first. Don't drink anything with sugar. Keep a snack handy in case you wake up in the middle of the night and eat it even if you aren't hungry. Eat every two hours even if you aren't hungry. Keep your meals and snacks small so you don't gain weight. I carry walnuts or almonds to snack on when I am out of the house.
If this diet helps, you probably have reactive hypoglycemia, low blood sugar. It might take up to a week to feel better. My hypoglycemia is worse before my period and when I am really stressed.
Giving up pasta, bread, etc. is difficult but you should know in about a week if it is helping. After you get your body used to eating like this, your blood sugar should become more stable so you can (carefully, and only occasionally) eat some high glycemic foods (bread, pasta, etc). Always eat protein and fat when you eat carbs.