What do it mean when a woman get a yellows discharge wit a fishy odor?

Hi, I i am new, but I have a question dealing wit the female l system. Well me and my ex have broken up for a few months now and we both had protected with other people while we was broken up but just recently we had unprotected . now she is complain she having a yellowish thin discharged and a fishy odor to her. Now she blaming me for it and think i gave her a STD but I was tested negative for any std. She was also tested be for she had wit me and she was negative also. I guess my question is.....is there something else then a STD that can cause symptoms like I describe earlier? pls help because I want to clear my name if its not a STD