you certainly need to be properly screened for diabetes. This would involve a blood test called Hba1c (average 3 month blood sugar), and you should also have a fasting blood sugar drawn.
The only way to know if you do or don't have a problem is to test your blood sugar. A home meter is typically not expensive and the results can be very revealing. I do suggest you look into getting a meter.
Given your weight gain (15 kg over ideal?) and lack of stamina recently, aside from having things properly tested, you need to make some lifestyle changes.
Lower carb diet (could be paleo style or modified mediteranean - eat the style but with less carbs) will be helpful for both weight management and blood sugar stabilization (useful whether high or low blood sugars).
Daily exercise will also be very helpful for your well being.
I agree with Super_sally888- getting properly screened for diabetes is important. Are you able to see another doctor? It doesn't sound like your are confident in your doctor- so looking for a second opinion would be a good idea!