What Causes Green Discharge?

Hi. about last month my doctor prescribed me some antibiotics and some cream becasue i had green discharge. i hadnt had sex for about a month but she said it was normal for woman to get that. i have only been with one guy fro 2 years and 7 months a month passed and yesterday the green discharge came back again. I hadnt had sex for about 3 weeks or so. What should i do? and why do i have this?