As your only symptom is of a fishy smelling discharge I shall comment on the possible causes for this. They are:
(smells worse after sexual intercourse, is called the Telltale sign)
-Imbalance in vaginal pH (from semen and vaginal secretion combination. IUDs have also shown to change the vaginal pH) -- so this can be considered and should be further investigated
Please visit your doctor for investigating your condition and appropriately getting treated for the same.
Hope this helps.