What Causes Pimple On The Penis?

Hi,

I had sexual relationships with less than 8 sex workers for the past 4months. Basically, it was just a short session less than 15 seconds with about 3 sex workers and it just usual vaginal intercourse with condom usage.The first was about 3months ago. The second was about one month ago and the last was on 4 days ago (saturday). The others 5 sex workers are just oral sex (they performed on me with condom usage too). On sunday, i noticed there is a small pimple at the neck of the my penis. Near the urethra duct, hidden under the skin. It is not painful. Just a yellowish small pimple. Is it possible to be STD or HIV early symptoms (herpes etc). There is not itchiness but just the pimple. I wouldn't notice if i did not examine my penis. Is it just normal bacterial infection or sebaceous blockage ? I did consult a GP on monday and he just gave a a white cream to apply. Till today (3rd day after application), the pimple is still there. No changes but the texture is no longer that yellowish. Not painful. Is it STD?Kindly reply. Thank you.