Hi My Doctor Put Me On Metformin Hcl 500 Mg

Hi my doctor put me on metformin hcl 500 mg 2 times a for pcos it has been great he retired and the new doctor one day changed them to metformin hcl er 500 mg 2 times a day I know they keep telling me it's the same I swear it's not the new one gives me bad gas and weight gain they say it's not possible I know my body can there be a difference in the two medications? Thank you Maria