Hi there. Fishy odor, when described that way, is often a sign of bacterial vaginosis. It has a distinct odor. Different than the usual smells of the vagina, it is quite unpleasant.
Discharge with BV is often milky, grey. If you are having this discharge and the odor, I agree with Jemma above that it is time to see your doctor. they can swab you and look to see exactly what it is (if anything). Antibiotics are given for BV.
good luck
It is common to have an undesirable odour after having intercourse.
This is why it is important that you go and have a pee straight away after having sex and wash as soon as possible afterwards. Wear clean cotton underwear ever day.
Should the strong fish odour be present at any other time, even with good hygiene, it would be likely that there was a bacterial infection.
In that case a appointment with the doctor would be in order for a examination and a swab to be sent to the lab to identify the problem. Sometimes a doctor will prescribe medication on symptoms presented.