How does one develop bacterial vaginosis?

Even though bacterial vaginosis is not considered a sexually transmitted disease you can acquire it sexually. 

You can easily acquire the infection from a new sexual partner(s) and from an old sexual partner who may have been promiscuous behind your back. Other risk factors for bacterial vaginosis include wearing an IUD, excess douching and recent antibiotic use. 

Some women develop the infection without any of the above risk factors- this is called bad luck.

I am a 24 year old female and during sex i have noticed a very foul odor (smells fishy) coming from the vagina. Do you know what it may be?

Yes, most likely you have acquired bacterial vaginosis. This infection is caused by an organism known as gardnerella. 

Bacterial vaginosis is very common and often associated with vaginal discharge and a foul odor. During intercourse, viable sperm can cause release of volatile amines from the discharge. Bacterial vaginosis is not considered to be a sexually transmitted disease. 

However, you do  need treatment. The disorder is easily diagnosed by looking under a microscope or by smelling the discharge (whiff test). The infections is treated with either clindamycin or metronidazole. These antibiotics can help resolve the infection in a few days. 

In the meantime, maintain good genital hygiene and avoid intercourse for a few days. Your partner does not have to be treated.