..., and currently in a relationship with a woman who has nonactive HSV2.

We are monagamous and have been sexually active for 4 weeks.  She is on Acyclovir for viral shedding suppression, and we do not use condoms.  I have discovered a small, somewhat mobile, nontender pea-sized subcutaneous nodule or cyst under the skin of the shaft of my penis.  I do not know how long it has been there, but my partner and I are sexually active and she is familiar with my genitalia, and never noticed the nodule before. 

I finished taking a 30-day round of antibiotics for prostatitis about a two months ago, which seems to have resolved.  Additionally, I have suffered from coital headaches in the last few weeks, which have cropped up and then resolved on their own several times during the last ten years. 

What are the possible/probable diagnoses for this symptom?  There is no pain, no erection problems, no discomfort with ejaculation (except the headaches, which seem to be resolving themselves).  Thanks for your advice.