Hi, and welcome (back) to our forum.
The stomach pain could be a whole host of conditions, the likes of IBS, or any of the bacteria of the gut, or diverticulitis.
You will have to see your GP about that.
Now, the second half of your question, is it is possible to have high then low blood glucose levels, if you have an intolerance to certain foods, and your hormonal response is not sufficient for the glucose derived, then the pancreas will produce more insulin. The problem in my case is there is too much insulin for the glucose and sends me low. On top of that, you may have insulin resistance and too much insulin being ineffective and making you gain weight.
Inflammation is a cause of type two diabetes.
It is not calories that you want to count, it is the carbs and sugar.
A low carb diet, with exercising is the best way forward.