480 HCG and no gestational sac

Help! I'm freaking out. I had my HCG levels checked today and they were only 480. I'm suppose to be almost 6 weeks. They also did an ultrasound and didn't even see a gestational sac. Just a thick endometrium. Has anyone ever had this before and it turn out okay? I'm getting my levels checked again Thursday, but I'm having a lot of pain and cramping and I'm really scared 😞😞😭