Are you taking any other medications alongside metformin?If so, it maybe possible that it may interact with another drug in the stomach, resulting in your discomfort.
Other verisons of metformin are available which you may be better suited to.
I'm assuming that you have been referred to a Diabetologist, who is determining the right medication and dosage for you. If not, ask your GP to be referred. Sometimes GP's don't really know how to find the best and most suitable medication.