chlamydia questions

i haven’t had sex in about 8 months and the guy i did have sex with was 100% a virgin but my doctor called a few days ago saying i tested positive for chlamydia but i don’t understand how an sti would last that long or even show up when no one i’ve had sex with in the past (years ago) tested positive. could the doctor have made a mistake? also i never had symptoms or any signs for chlamydia