If i have sex unprotected can it cause yellow discharge?

No: yellow discharge indicates INFECTION..you need a GYN evaluation ASAP Hope this is helpful Dr Z

Infection, STD?: Sex itself doesn't cause vaginal discharge. Colored discharge (yellow, creamy, etc) suggests infection, including STDs like gonorrhea, chlamydia or trichomonas. Possibly a vaginal infection like bacterial vaginosis, but this usually causes odor and discharge is white. Get examined and tested ASAP. Good luck.