A discharge with a fishy odor is usually caused by a condition called bacterial vaginosis
(BV), which is not really an infection but rather a change in vaginal bacteria
from the good ones to others that aren't so good. It can be diagnosed using a microscope and is treated with certain antibiotics.
When there is a fishy odor after intercourse that only lasts a day or so, that's also related to the type of bacteria present in the vagina
, where the alkaline pH of the semen brings out the fishy odor from the bacteria. As long as it only lasts a day, it doesn't need any treatment, as it goes away on its own.
BV is more common in women that douche, since douching washes out the good bacteria as well as the bad. The good bacteria are needed to maintain a normal vagina. So not douching will usually be helpful in the long run.
It's quite unusual to have a yeast infection without itching.
BV has a tendency to come back, and this can be a difficult problem. Usually it's best to work with your doctor to figure out the best way to deal with it, including certain antibiotic pills, creams or suppositories. Sometimes it's helpful to also use lactobacillus acidophilus suppositories to help maintain the good vaginal bacteria.