It could be an infected hair follicle, but you should get it checked out since it's not getting better.
Chances are that it's not herpes because it's so small and there's only one, and it doesn't sound like a genital wart. If it is a wart, those take a few months to appear, so depending on how recent your encounter was, it might not be from them.
If the doctor thinks it's herpes (and it really doesn't sound like it from your description), make them do a type specific culture on it, preferably a PCR swab.
Let us know what happens.