What Causes Vaginal Odor After Sexual Intercourse?

My vagina takes on a distinctly fishy smell after sex with my boyfriend/the day after. This has only started happening in the last week or so, we have been together for nearly two years. he has recently changed the soap he has been using? I also came of the pill about 3 months ago could this be a reason behind it? I feel like i can smell it through my clothes during the day.