What Causes Greenish Vaginal Discharge?

hi, i suffer from a watery green type discharge i recently went to my local GUM CLINIC as i had symptoms which involved me bleeding a few days after sex and occasionally severe burning downstairs. i had full swabs taken they even examined my womb and ovaries and all results came back negative. they gave me some tablets to take which would flush out ne bacteria etc and they claimed it may be pelvic inflammotory disease however when all results and swabs came back i was clear of everything. what could it be as i am still getting the discharge? and what should i do next?