Hi Cosy - you have a point there, I have noticed this happening with some medication in the past, I once took some tablets back to the pharmacy but was told at the time that it was only in my imagination or that the side effects are from something else, that was some years ago though and later I insisted on being given one brand only and was ok after that.
I think that all generic drugs have to have the same active ingredients, eg 500g metformin is the same but whatever else is added to the metformin may differ between all those manufacturers and this can cause different side effects? Ask your pharmacist/doctor about this.
All the best
Karen