Abnormal pap smear..?

i was told by my doctor that i have an abnormal pap smear but there is no hpv detected at this time or infection. She also said that every now and then there will be cells abnormality, that i should come back for another pap smear six months from now. what could this be? my boyfriend and i have been together for 2yrs now and he was in the military where they check you for std's well he had no std's meaning if i had anything he would have gotten it (no condoms at the time) but now we use condoms as well as birth control pills. This is my first abnormal one ever. I've had normal ones being with my boyfriend that i am with today. I've had regular pap smears every year and then all of a sudden one abnormal one. what could this be? it's driving me crazy and i'm a worry freak!