Sometimes I have yellow discharge w/ a smell but it's not a strong or fishy odor could I have a STD or just a yeast infection?

Bacterial Vaginosis. Sometimes disturbances to the natural bacteria in your vagina (douching, irregular bleeding, unprotected sex, etc) can allow abnl bacteria like Gardnerella to overgrow and cause a fishy odor. It's not sexually transmitted, but can be treated w/ Metronidazole(Flagyl)/Clindamycin/Boric Acid. See your GYN for a prescription and be examined for other potential causes. Nothing urgent/dangerous though.
Yes could be an std. Please refrain from sexual activity, get tested and get treated. Make sure your partner(s) are tested and treated or you can keep reinfecting each other. Remember that if you are at risk for one std you are at risk for all of them. Get tested for syphilis and HIV also. Good luck and stay safe.