I am on Metformin 500mg tablet twice a day.

It has given me diarrhea. Sorry TMI.
:hugs: that you are not feeling well. Defaintly take the advice of taking it slowly. I started on 1 tablet (500mg) of metformin and was fine. I went up to 1 and half, then worked up to 1000mg (2 tablets) yet I wasnt well in other ways and as my FS wasnt too long away I decreased. I had the tummy upset, felt extremely tired as well.
Ive still been on 500mg and now back up to 1 and half tablets and are having tummy upsets. Yet previously on 1 and 1/2 I was fine. Take a note of what foods you are eating and whether any of those make things worse for you.