Hello,
It could be an infection. The most common cause is vaginal bacterial overgrowth known as bacterial vaginosis, which has a "fishy" or "musty" smell.
"Fishy" smells can also result from vaginal infections, such as trichomonas vaginalis. The vagina could also smell "yeasty" because of vaginal yeast overgrowths - yeast infection
In about 25% of women, BV symptoms resolve on their own. you can use lactobacillus (as in yogurt) to help restore the balance of vaginal bacteria, either along with or instead of antibiotic treatment
If it isn't that and it doesn't clear up quickly, you need to see a gynecologist.