It's probably nothing to worry about, but since you are concerned, why not make an appt with your derm and ask specifically about it? Mention that it's been there awhile, and if you aren't satisfied with her answer about what it is, ask her to biopsy it.
Chances are the doctor would have been suspicious and mentioned it, but there's nothing wrong with putting your mind at ease just in case she missed it.
Let us know what happens. :)