Hi shadwell,
It's really frustrating when your efforts to exercise are rewarded with a rise in blood sugar, isn't it? :evil:
I know you're t2, whereas I'm t1, so the reasons behind it can be different in each case.
Exercise such as swimming or running will cause a release of stress hormones (adrenaline, cortisol etc.) which cause the liver and muscles to convert glycogen to glucose, just as you said. If I start a run or cycle a few hours after eating, there is no fast acting insulin in my system and the long acting can be insufficient to cover this rise. My solution has been to always eat before I start so I can recruit the fast acting stuff to act against the rise in bs. If I inject a little less, or eat a little more, I can keep my bs pretty stable.
A t2 making a lot of insulin (all do at first) will probably escape this rise, and may instead find their bs falling quite low after exercise. Your experience might suggest that your insulin levels are lower than this.
You could ask your gp or clinic to test your serum insulin levels (an inexpensive test) to see where you are with insulin production levels. Otherwise, the other ways in which your bs will come into line are if they aren't elevated before you start, and if your exercise is sufficiently prolonged to actually start bringing it down again.
I hope that helps?
All the best,
fergus