This sounds like a sebaceous cyst that became infected; or garden-variety folliculitis or acne. It definitely isn't herpes, which wouldn't last a month then come to a head. But of course I can't be certain; for a definite diagnosis you need to see a healthcare provider. You definitely need to do so if the lesion persists, gets worse, etc.
As a sexually active young (I'm guessing) woman who has changed partners, even infrequently, you should have annual testing for common STDs, such as chlamydia and gonorrhea. If that hasn't been done recently, this would be a good time; the provider can also check out the lesion you describe.
Good luck-- HHH, MD