One thing you can do is this
http://www.alt-support-diabetes.org/NewlyDiagnosed.htm
you can buy a meter at your local pharmacy with a small quantity of strips included.
Try testing before you eat then 1/2 hour, 1, 2 and 3 hours after.
It SOUNDS from what you describe that you MAY be suffering from Reactive Hypoglycemia, which is sort of an early form of diabetes: you lose your first phase insulin so your BG jumps up after eating, then your pancreas notices and dumps a load of second phase insulin - then fails to stop, so your BG goes down below normal.
This can be an absolute ******* to get diagnosed if (like me) your fasting BG is normal and two hours after eating it's also back to normal again, you need to look at the 1 hour and 3 hour numbers to see what's really happening. A Glucose Tolerance Test should show it if you can get one.
The "treatment" is exactly the same as "real" diabetes, reduce the carbs so you no longer spike high after eating, which will also knock out the lows.
The alternative is to keep on until after several years you have actually disintegrated so far that they actually diagnose you with diabetes.
This is NOT a diagnosis, just a suggestion of a test that can be done easily and which may provide you and your doctor with useful information.