It is just a co incidence that you are experiencing high levels of PP blood sugar levels after consuming METFORMIN
This seems to have happened due to progressive loss of beta cells of pancreas
resulting in loss of control on your sugar levels and ,in no way due to consumtion of METFORMIN.
It simply means that METFORMIN in present dose is ineffective in controlling the sugar levels and you have to either increase the dose of METFORMIN or you have to change the type of anti diabetic drug.
It is a natural course of progression of diabetes,where oral anti diabetics tend to loose their efficacy gradually and this happens due to progressive destruction of beta cells,as explained above.
Consult your doctor and get the prescription revised.
Make sure to keep a strict control on your diabetes as uncontrolled diabetes
results in various micro as well as macro vascular complications in long run.
Observe proper dietary restrictions and do regular aerobic exercises
for 45 minutes a day.