It may be herpes, an outbreak doesn't always appear as clusters or like many of the photos on the internet.
I suggest that you go to a doctor. Unfortunately if the sore is not swabbed within the first 24-48 hrs after appearing there is a chance for a false negative. You can also ask for the IgG herpes type specific blood test but it normally takes up to 4 months for the antibodies to be detected in the blood.
I wish you the best of luck