Metformin tends to have this effect on some people. I'm one of the lucky ones like you, I moved into the loo for weeks before going off it. Metformin doesn't have a very big impact on bloodsugars as it is (It's an appetite-suppresant and tells your liver to dump less glucose into your bloodstream, but it doesn't do much for what you ingest carb/sugar-wise). If it's more than 2 weeks and you're still a geiser, you can try the slow release metformin, which gets tolerated better by some folks. Or make sure you have it with a meal, rather than on an empty stomach, I heard that could make a difference too. Me, I got shocked into changing my diet because of how I responded to metformin, and haven't looked back. Diet-controlled for 2 years, non-diabetic levels with no T2 meds at all, and hopeful to keep it that way. Maybe Low Carb/High Fat is something to look into?
Good luck!
Jo