Clare is quite right - a singualr blood test is very inconclusive.
You need to ask your GP for a glucose tolerance test. You have to fast for 8hrs then they'll give you a bottle of this syrupy stuff to drink. They take a base line blood sample prior to you drinking that, then they take blood every hour for 3 hours so they can plot a graph of your blood glucose and insulin levels.
After about 2.5hrs I was feeling pretty ill, hands shaking, very very angry at absolutely nothing and generally a bit queazy.
Again like Clare says if you change your diet its nothing at all to worry about