It's my belief that Metformin shaves the edges off your blood scores, rather has a significant impact. Indeed, that's why, in my view, we see so many T2s going low carb - diet is our primary and most significant weapon against our diabetes.
I have see other pregnant T2s post about erratic blood sugars, which I have always concluded could be due to fluctuating hormone levels as the pregnancy gets going and "feeds" the body's other mystical requirements to sustain the foetus, placenta and all the other impacted "bits".
Going back to Metformin, one of it's primary impacts is to improve insulin resistance, so it could well be that your body is utilising the insulin you have been injecting more effectively or more quickly, bit whatever the reason is, it would seem to make sense to have a chat with your pregnancy diabetes team to work out the best way forward for you.
As I'm neither T1, not have I ever taken Metformin, please don't act upon anything I've said. I'm merely letting my thoughts loose via the keyboard.
Good luck with it all. Exciting, and no doubt scary times, in equal measure!