Couldn't confirm at 5 weeks

Went to the emergency room last night because I've been having pain (feels like bad menstral cramps) and I am afraid I have an ectopic pregnancy. The ultrasound tech did a vaginal US and said "it looks like there is a sac beginning to form". They couldn't confirm a pregnancy. 😞 Now they want me back in 3 days to make sure my hcg levels are still rising and my pregnancy is viable. Has this happened to anyone else? Thoughts?