Herpes cold sores generally affect the middle of the lip, not the corners. It could be a pimple, it could be an irritation. Try putting bacitracin or neosporin ointment on it, don't mess with it which might confuse the issue, and show a doctor as soon as you can. Not that it's an emergency, but you can only diagnose herpes with certainty when it's there. If you wait till it's gone, you'll be left in doubt, and I don't think you need to be.
Best.
Dr. Rockoff