I am 6 weeks pregnant and an ultrasound showed a gestational sac and yolk sac, but no embryo, does this mean i could still have an ectopic pregnancy?

Very unlikely: I presume the gestational sac and yolk sac are located in the uterus, in this case, the chance of a concomitant tubal pregnancy is extremely small, almost negligible. Hopefully, you do not have any bleeding or cramping, in which case, the ultrasound may be repeated in 7-10 days to determine viability (presence of a fetal pole with heart beat). Best Wishes