I'm a 23 year old female and I've had a very painful yellow sore on the left side of my tongue, slightly towards the back. It's located right on the side - not on the top or bottom side of my tongue. The sore itself isn't huge (maybe less than half the size of a pencil eraser), but the area it is on is swollen enough that it hurts to talk or just move my tongue in general. I've had canker sores before, but I usually get white ones with a red border, and this one is just yellow - definitely secluded but no defined border. Does this sound like it's just a canker sore or does it sound like an STI?