Welcome to mothering. Have you taken any medications (such as fertility meds) this month which could have caused a cyst? Cysts may also appear without meds. Are you sure you certain you ovulated when you normally do (any stress, good or bad, can delay ovulation)?
My periods are like clockwork, but I've had an odd month. I know for me, when my period come late I feel physically awful/sick until it comes. After 16 days, a hpt is going to be very accurate, so it's unlikely that you're pregnant. If it were me, I'd make an appt with the gyno to find out what may be going on. They will probably order a bloodtest to r/o pregnancy.