I too had a really bad reaction to bog standard Metformin with no warning random diarrhoea which meant I wouldn't go out of the house. I told my GP very bluntly about the effect it was having on my life and demanded the SR version. He was very apologetic and I got the impression that GPs are told to try the standard one first.
My GP advised me to build up to my full dose. From my experience I would suggest that Metformin is systemic i.e. you need a constant supply to keep up the level in your body and that when the level drops, probs can also occur.
Sometimes in the past when I have overindulged in carbs (for me), I have thrown up rather than had diarrhoea, which isn't nice but doesn't last hours.
If I am late taking my evening dose, I can get diarrhoea a couple of days later so I learned that in those rare circumstances a couple of generic Loperamide from Lidl slows my digestive system down enough to avoid it. My GP does know that I do that and very rarely.
I too would suggest talking to your GP, our digestive systems are different !