The only way to know what's going on is to be seen by your medical provider. It could be a lingering yeast infection, or something else.
Have you ever been tested for herpes? If not, now's a good a time as ever. As well, be sure your partner in fact was tested; most STD screenings do not include herpes.
Oh and i meant to mention that is more like a blister it's just red and there's only one.