It can be side effect sadly to say (diarrhoea, bad diarrhoea!) with metformin. I had normal metformin to start with, but because of the effect it had on my bowels I was then given slow release metformin, but they too gave me the same awful effect. Some people can take it with no side effects, normal or SR, but I can't take either type. I saw the dietician a few times who kept on telling me that it was my diet, not the medication, but the diarrhoea only stopped when the diabetic nurse agreed to me coming off the tablets a year past Feb/March can't remember which month. I've hardly had a sick day from work since coming off the medication, before hand I often had to go home because of the effect the tablets had!
Go back to your doctor and explain how you feel and ask to be changed to either slow release (they cost more I believe, so the Dr's are not keen to give them, so the nurse told me!) or another type of medication.
Look on here for information on the low carb high fat diet (LCHF) which helps type 2 diabetics. I was lucky as my Diabetic nurse told me all about it and then I came here and looked it up. It has certainly helped to lower my BS levels. But read up all you can about it before you decide which way to go. Lots of good information on the net about it.