Possibly look into dysautonomia or POTS(postural orthostatic tachycardia syndrome). My son had the same symptoms and it took us a really long time to get him properly diagnosed. The blood may be pooling in the stomach, causing abdominal pain and dizziness since it isn't making it back up to the brain.
The autonomic nervous system regulates all functions in the body that are involuntary(heart rate, blood pressure, digestion, body temp,etc). For my son, it started out with abdominal pain and nausea. Then dizziness and headaches and several other symptoms eventually added on. (hot spells, hand tremors, migraines, weak legs).
There are 2 good really websites for info dinet. org and ndrf. org.
I don't know what you have, but my husband had the same symptoms and they were relieved with a gluten free diet (no wheat, no barley, no rye, no oats). You can look the diet up on the internet. Give it several weeks and see if it helps.
Good luck!