Freaking out (no gestational sac)

Ra
Went in to the ER yesterday for what felt like an ovarian cyst. I wasn't sure if you could get them while pregnant but I am supposedly only 5.5 weeks along. During the ultrasound they confirmed cysts but couldn't find a gestational sac. They said that's not uncommon and I may not be as far along as they think (I ovulated the week before period was due which in theory coyld have delayed it which would make me only 4 weeks). My HCG was 275. They said they have to retest my HCG tomorrow and as long as it has doubled, they aren't going to worry. If it hasn't, it may be eptoptic. I'm a mess. Has something similar ever happened to any of you?