Preop protein shakes sugars very low

I just figured out that I can't take my metformin (diabetes medicine) while I am on this protein shake only diet.  My sugars are dropping big time.  Should I have known this?  Makes sense to me now.  Why didn't I think about it before it happened?  Felling kind of stupid.

Good news, I think I will be done with diabetes medicine from now on.  I remember I never took it after my last surgery until I started gaining my weight back after having my kids.