Been overthinking since

Last night I went to the ER for cramping pain and headache, I was told that my blood pressure is high. I’m suppose to be 5 weeks and 4 days (yes I’m extremely early) they did an ultrasound and told me my sac was empty it isn’t in my uterus, fallopian tube, or ovaries. I feel like they should’ve seen something by now. They told me sometimes it normal not to see anything yet but also retest to see if my hcg level rises in 48 hours and to mentally prepare myself in case it’s an ectopic pregnancy. Is it normal not to see anything at this time?