5 weeks 5 days no gestational sac :(

Madds

I just had an ultrasound after having low hcg levels (20dpo=195; 22dpo=299; 24dpo=524; 26dpo=855) and there was no sac seen on the ultrasound. Is this normal? I’m freaking out. Has this happened to anyone? Why wouldn’t there be a sac? Doesn’t this appear really early? Any experience with this?