What Causes A Canker Sore On The Outer Lip Of Vagina?

i have what looks like canker sore my outer lip of my vagina. it stings and is red and soft and if I wipe it I will see a little discharge on the napkin I believe pus. I recently had a Brazilian waxing and the next day i had unprotected sex with my boyfriend. My menstrual cycle came on after i had sex and couple days later i noticed the sore. What could it be