CAN ANYONE HELP?

I had a doctors visit 2 days ago because of light bleeding on and off. When I went they tested me there and got a positive test but when they did the ultra sound found no sac. There was blood in my uterus and that was all that could be seen. I am around 6-7 weeks if I calculated right. My hcg levels were taken and I go back tomorrow to hope they have doubled. If they have I may have just calculated wrong and it was too early to see. Has anyone experienced this and had a healthy pregnancy? No sac but HCG still rising?