Ultrasound showed sac but no baby

I had my first ultrasound at 8 weeks which only showed a sac. The nurse said that i am probably more like 5-6 weeks and to come back next week for another scan. Im really worried. Has this happened to anyone and you have gone back to find a baby in the sac? I do have a tilted uterus which i never mentioned to the nurse. Also my cycles can be anywhere between 30-38 days.