Gestational sac, nothing else - 5 weeks

Hi! I went to the dr yesterday due to cramps and lower backache, which made me nervous. My boobs seem to grow and shrink, nipples are sore and then not, and nausea comes and goes since I️ found out last Tuesday. They said my hcg was 5,610 and they only saw a gestational sac, nothing else and were concerned they should have seen more at this stage (5 weeks, 4 days). They want me to come back in 48 hours to see if the hcg rose or not. Has anyone ever experienced this? I also have really long cycles (34-36 days) and short periods (3-4 days). I’d greatly appreciate any feedback, as I am extremely nervous now. Thank you!