Hi,
I agree with what DCUKmod has said, but would just like to add:
How long have you been back on the Metformin?
We often get people posting who have been told by their docs to take one tablet a day, and then, if they don't get a tummy reaction, to add a second tablet after a week or so, and on to a 3rd tablet a week after that, until they have reached the prescribed dose. If they get tummy problems, stay at that dose for a few weeks or, until the problems resolve, then add in the next tablet.
Now, obviously, if you are just on one tablet, then most of that advice doesn't apply to you. But it does suggest that adjusting to Metformin can take a few weeks.
And I would suggest that if you haven't adjusted by the end of 3-4 weeks, then don't suffer in silence. You have enough going on with your other health concerns at the moment. Go to your doc as DCUKmod suggests, and explain. I would imagine that your body is much more reactive and sensitive at the moment, so you may find the slow release Metformin much more bearable.
Hope the chemo goes well, and that you feel much better afterwards.