According to the symptoms you report (smelly yellow vaginal discharge, irritation…) it seems very likely that you are experiencing vaginal inflammation caused by an infection (colpitis). A yeast infection is not considered a sexually transmitted disease although the infective agent (Candida) can be imported by unprotected sex. This is because Candida normally lives in the vagina but local immune mechanisms prevent it from causing infection. During some instances (pregnancy, antibiotic and corticosteroid therapy, birth control pills, diabetes…) the immune system becomes weak and Candida can become pathogenic, causing infection.
Unprotected sex without a condom is of great importance for diagnosing re-occurring vaginal infections, but not in case of Candida-infection. You can go to your gynecologist and request a vaginal examination, a vaginal smear for identifying the level of vaginal clarity, and a Pap-test.