Last Night I Experienced An Extremely Itchy And Red Vaginal

Last night I experienced an extremely itchy and red vaginal opening and noticed a slight burning while urinating. Out of exasperation I scratched the area quite hard. This morning and throughout the day I experienced a thick brown discharge similar to dried blood. As the day went on it lightened and has since seemed to dissipate. While I initially thought this was a yeast infection, the discharge has me worried that this may be an STI. I am 22 years old, am on day 12 of my pack of Yaz birth control and had intercourse without a condom a little over a week ago. What do you think is happening?