Umm, do you have sex? I know when I had sex with my boyfriend he was a bit rough and that caused them to be sore all the time. Might also your period, homornal imbalance, birth control.
Sorry to say this but just because you were only with your boyfriend doesnt mean your boyfriend only been with you. Maybe you both should get tested out. But it might always just be a pimple, I never got a pimple there before tho