Does Insulin Increases Weight?

Last year, I had an Aortic Aneurysm. The Surgeon used the "David" procedure, which replaced my heart aorta with an artificial tube, and re-attached my own valve. After the surgery, I was 221 lbs (I suffered a stroke and was in a coma for 7 days). I am now, 250 lbs. I am not an over eater, in fact, I have maybe a granola bar for with coffee for breakfast, a small soup for lunch and a fairly balanced supper. I am 62 years old. I have spikes in my sugar levels, as sometimes I get the shakes and one time had my sugar drop to 30. I have had my A1C checked several times, and am not diabetic. I have read something of late, that my Insulin function may be the cause for the weight gain. My thyroid has also been checked and is functioning normally. Do you have any suggestions for the weight gain in the last 18 months? My Dr.(cardiologist) suggested more exercise? Thank you!