If you have poor release of insulin, then after exercise the blood glucose
can rise. While exercising the brain gets a signal to increase blood glucose
to meet the extra body demand. If this increased sugar is able to trigger an increased insulin response, the blood sugar gets lowered. Otherwise, it remains high.
You should get GTT or glucose tolerance test, fasting blood sugar, post prandial sugar done and glycosyltaed hemoglobin measured to know whether you have diabetes or not.