hi could be just a missed period women do get these from time to time could be that your stressed that will do the same could your under weight or overweight perhaps you have been overdoing exercising this will do it as well. but if you think its an std or sti just go to an std/gum /clinic and get tested this would find out for you and the staff are helpful. do hope something here helps. good luck
missed my period and am not pregnant could gonorrhea be the cause