It is normally due to an imbalance in vaginal bacteria. The bad bacteria takes over because of the friction, heat and they feed on his semen. That will cause the fishy smell. Sometimes we also get that around our periods for the same type of reasons.
You need to see your doctor to make sure you do not have a permanent bacterial infection like bacterial vaginosis (BV). It is easy to treat, and it is not sexually transmitted. BV is a very sneaky infection that can go years and years without detection.