Go to the doc. You need to be checked for an infection. You don't say if you're sexually active, but even if not, there are a lot of things that can cause this. It is from an excessive amount of bacterium, which can even be hormonally caused. You need to have a culture and/or pap smear done to find out which bacteria is causing the smell. R u on any medications? Some oral meds have a negative effect on your immune system and may be allowing bacteria to grow out of check.
As a last thought, it may help if you try to keep your vagina, inside and out, dryer by using a tampon. Just remember that you prob. Should change it every 3 - 4 hours.
I say to keep it dry as bacteria needs moisture, and most all foul odors are a bi-product of bacterial growth.