Hi feebiecat,
Metformin is a safe medicine and I've used it for nearly 6 years without problems. Many people get the "tummy grumblies". Most adapt fairly quickly. Some never get them. I didn't.
However, I can see you being worried, because you are very young.
Firstly are the certain you areT2?
That seems to me to be the first thing to establish. If you are not, Metformin won't help.
If you are, getting your weight down into the low part of the normal range, can sometimes abate symptoms for a while. It's VERY hard to do I know, but Metformin helps some people.
It might be worth discussing Byetta with your doctor. You are not an obvious candidate for it, but you have many years ahead of you which might put a differnt slant on things.
Still neither Byetta or Metformin will work if you are not T2.( you say low carb didn't work for you)
My husband developed T1 at about your age.