What you have is a common effect of metformin. Normally wears off eventually, normally starts straight away, but in your case delayed start.
You could go back to the GP and ask for the "slow release" version of metformin (metformin SR) which doesn't give these side effects.
You say you're going back for tests at the doctors, but you have obviously realised with your readings that you ARE diabetic?
Goog Luck, and stick with the diet. It will pay off and do much more to control the blood sugars than the metformin will.