check into trichomoniasis. An STD that sounds like what you are describing and its curable.
Yellow discharge and bad smell most likely equals a bacterial infection. See your medical provider!
The only way to knwo for sure is to seek medical advice, and as soon as possible as whatever it is could be detrimental to your health! Lots of conditions apart from and including STD's can cause discharge so really you need to be tested!
Good Luck.
Mel.