If there is fishy smell down there, it could be due to bacterial vaginosis. This could occur in women who fall in the age group of 15-44. Overgrowth of bacteria causes BV.
Though it is harmless, it is better to restore the pH of the vagina so that the smell disappears. Consult your doctor to get an antibiotic. Though yeast infection doesn't give a foul smell, the discharge which resembles cheese could even cause itching sensation.
Burning sensation around the privates and pain while urinating are some signs to recognise the yeast infection. The material of your undergarments could cause this type of smell.
The smell comes when sweat gets stuck down there. If you have gone for a workout with undergarments that are not breathable then it could cause musky smell. 