Clia,
You say you can keep your blood glucose levels "good", but do they significantly drop after activity? Do you check you sugar levels at those times?
The reason I'm asking is a lot of post-op gastric bypass patients feel very fatigued after just a little activity, but when their sugars are checked, they haven't really dropped much. The bottoming out feeling is from muscle and cardiovascular fatigue. Most bypass patients have not been very active before surgery, and then the surgery saps what reserve they have. So, they really are starting at the bottom in terms of getting into shape. Any activity often fatigues them very quickly, but doesn't really cause a drop in glucose level.
The body's physiology is such, that it is designed to regulate the over all blood glucose. Even in periods where a person does not consume enough calories to cover physical activity demands, the body will covert fat to glucose, to cover the need. If fat is not available, such as in starvation states, then muscle will be converted.
So, if you are not maintaining a fairly stable glucose level, you might need a metabolic evaluation, to find out why. And to try to find a solution for it. Because, if your sugars are bottoming out, taking in high calorie foods is not going to stop that. High protein diets will help to maintain a good glucose level, but it is not converted very quickly, so it can't react to a sudden drop. High fat diets often are not digested well in bypass patients, but fat has the most calories per gram of any food.
Again, if your sugars are bottoming out, despite an adequate consumption, then it should probably be evaluated by a dietician or physician. Good luck.