Is this normal? Has this happened to anyone?

I got a + on an ovulation test on Nov. 14. I went to the doc on Nov. 28 for blood work and found out I was preg. (HCG level 13) I went to the doc on Dec 22 for an ultrasound and it shows a sac and it measures 5 weeks. I had more bloodwork done and the doc said that my blood work matches the ultrasound and I'm 5 weeks. I'm just worried. Could this be normal? I was thinking that I was further along than 5 weeks. Has this ever happened to anyone before?