You can often feel lethargic and sluggish with high blood glucose levels. There may also be thyroid elements in the mix as well, but obviously until the results have been discussed - you won't know...
You may also wish to check your BG after meals to see how your BG is reacting to the carbs. Fasting BG is a tiny proportional of out overall BG profile. When you test after meals, you can start to adjust portion sizes and/or avoid foods which give you the highest numbers. Once your overall control tightens, lethargy is more than likely to subside
Ideally, you'd like to test between 90 mins and 2 hours after starting your meal. If you want to see just how high the food makes your BG spike, you could try testing at the 1 hr mark instead. It's up to yourself, but you can read some more on what your BG should be here:
http://www.diabetes.co.uk/diabetes_care/blood-sugar-level-ranges.html