Is this just a pimple or could it be an STD?

For about 3 or 4 days now, I have had what looks like a pimple on the inner part of my outer labia...if that makes sense. It is raised and red and very sore to the touch and has been getting bigger/more sore everyday. It hurts when I walk as well because it's kinda in a crevice (TMI sorry!) so it rubs when I walk. Is it possible that this is just a pimple or does it sound like it could be herpes or something? I did have unprotected sex with my friend about 12 days ago but I have given him oral sex before and never got sores on my mouth or anything so that's what makes me think it isn't any kind of STD because I would have gotten it by now, but I'm a hypochondriac and still worry about stuff like this. Also from what I've read about herpes, the sores are usually more flat and leak fluid, which mine is not, and usually only last a few days and mine has been around for almost 5 now. Thoughts please!