What Causes Fishy Odor And White Discharge From Vagina?

Hi my name is Megan I am in serious need of answers. My vagina has a VERY strong fish like odor. I have used Monistat twice now. The first time I used it the smell went away for a week then it came back so I am on my second dose but the smell is slowly returning. I also have a very thick white discharge. I have always been a clean freak so I am very worried. I have always wore condoms with sex til bout a month ago like two times. Could this be an STD? :(