What Causes Fishy Smell After Intercourse?

Im 5mos pregnant and just had a check up for yeast and bv the doctor saw no signs to any one but tested me for stds and I have not got a phone call saying I have anything. But I let my boyfriend cum inside of me and the next day I smell a fishy odor why is that and can sperm cause a woman to smell and for how long dose it take to clear?