Spotting and ultrasound

This morning after I showered I had a small amount of spotting so I went to the doctor. The ultrasound said that according to the sac size I am a little over 4 weeks (based on my last period I thought I was at 6), but my HCG level is at 11,134 which seems further along to me. They said I have to go back in another week for another ultrasound to make sure the sac is growing. Has anyone experience this before or have any advice? The spotting is what really scares me!