It is nearly impossible to determine any characteristic or a Sexually Transmitted Disease (STD) or Sexually Transmitted Infection (STI) because of the many different manifestations it can take. The infections are still fairly new and not all causes are fully understood by medical personnel.
Therefore when a STD or STI is suspected, the Center For Disease Control and Prevention requires that all sexual partners be tested. Not all signs of infection appears in all indivisuals so it is a possibility that you may have had this way before your previous sexual encounters.
Herpes Simplex Virus 2, for example. HVS-2 is transmitted through sexual contact; symptoms may begin as a series of mostly painful blisters on the penis and inside the vagina and cervix.
The blisters usually dry up and vanish within a week or so, even without treatment, but unfortunately that is not the end of it. Months, sometimes years, later HSV-2 will recur, striking with the same virulence as before.
Nonspecific Urethritis is a catchall veneral disease, other that gonorrhea, that causes inflammation of the uretha. Granuloma Inguinale is a slow-growing veneral disease which is often passed as a secondary infection; First appearances are pimples, blisters, or nodules that turn into beffy red ulcers with a slight purulent discharge.
Lymphogranuloma Venereum (LGV)is another world-wide veneral disease caused by a virus and can begin by small blisters or pimples appearing in the genital area, which can also heal quickly. The lymph nodes in the groin later swells up within about 3 days - 3 weeks.
If you have gotten no medication or antibiotics from you Doctor, you can feel free to visit a local clinic or your local Health Department. If you have no condoms, you can get them from one of these two place too, for free.
Hope this answers your question!
REFERENCE(S)
1. (CDC) Center Of Disease Control an Prevention, 2006.
General Information - STD information from CDC