I have white discharge with a fishy smell and sometimes I have itchy could I have a STD or a bad yeast infection?

Seevmd. Examination by doctor is the best way to make proper diagnosis. Please see your doctor.
Bacterial vaginosis? Vaginal yeast usually causes little or no discharge or odor; itching and irritation are the main symptoms. The main cause of strong (often fishy) vaginal odor, with or without discharge, is bacterial vaginosis, an overgrowth of mostly normal bacterial. The STD trichomonas causes also causes discharge and odor, and gonorrhea and chlamydia are more common in women with BV. Get checked for STDs ASAP.