Did your pharmacist change the brand of Metformin with your last prescription?
The pharmacists sometimes shop around to get a better deal and buy a less expensive brand, in order to save money.
Sometimes the ingredients/proportions are slightly different in different brands, and that can kick off a reaction.
Alternatively, something else may have upset your stomach slightly, which has made it more sensitive to the metformin.
OR, if you missed a dose or two, your tolerance of the drug may need to build up again.
Hope that helps.