As you know, there isn't a ton of research on it yet. I think Hale considers it quite safe, and if I remember right way more crosses the placenta than gets into breastmilk, so it seemed to me that if I was going to take it in pregnancy, I might as well continue while nursing.
I was on metformin my first trimester w/ my now 3 year old. I went back on met when he was about 2. My endo did not like RXing it to nursing women but I told her we had weaned
after doing my own research and feeling okay w/ the safety of it. I wanted to see if it would give me my period back...it never worked when we were ttc DS but I went on it at a higher dose and wanted to give that time before having to decide whether to wean to start ttc# w/ injectables. 3 months after starting met, I was frustrated to still not have a cycle. I was about to induce a period with progesterone but decided to take a HPT just because *I should* ...never expecting a BFP
(DS was an injectables /IUI baby).
This time around I stayed on met my whole pregnancy and am on it while BFing my almost 3 month old. I see a practice of midwives, and both they and their backup OB support my using met while BFing. I haven't seen any ill side effects while using it and BF. Obviously you have to do what you are comfortable with. If she's also drooling a lot, the increased pooping could be from teeth as you said.
Probably not very helpful but I wanted to answer since I'm a mom that uses met too.
