Isla, It is normally a sign of a bacterial infection. The most common is bacterial vaginosis (BV). Your doctor can treat it with anti-biotics.
BV is not a sexually transmitted disease. Although it is not quite understood why you get it, the main contributing factor is the number of sexual partners you allow to ejaculate inside your vagina. After the 3rd one, your chances increase dramatically to get BV.
From an Oriental Medicine perspective (acupuncture, Chinese herbal) this symptom does not necessarily correspond to an infection, but this depends on how strong the odor is, and whether or not it can be considered a "light" or "fresh" seafood smell, or if it could be considered "foul" or very strong. A practitioner can help you.
A less foul fishy smell is usually a sign of cold or cold-damp accumulated in the urinary bladder or liver channels affecting the vagina. Other signs & symptoms can be used to verify such as cold in the lower legs or feet, cold urination, a lengthening of your cycle, clear or white discharge, and quite a few others. The treatments are helpful not just for the symptom but also for the quality of your life... the treatment almost always makes you feel better in addition to resolving the symptom. Antibiotics, in this situation, can actually feed the problem by introducing more cold energy and further damaging the digestion (which is a fundamental cause for the cold accum in the first place). Use antibiotics ONLY in the case of a verified infection, not just a symptom that seems abnormal -- far too many MD's diagnose and treat like this.
You will need to visit a quality qualified practitioner if you wish to see what Oriental medicine has to offer. Self-diagnosis is not an option for you... you need a second set of eyes, as it were, to gain a clear perspective of your situation. Good luck!