Brown spotting at 7 weeks...ultrasound measuring behind?

On Sunday afternoon I had some dark brown discharge in my underwater. From then until Monday afternoon, when I would wipe I would have a small amount of brown to dark brown spotting.

As this is my first pregnancy, I went to the DR and got an ultrasound. I was exactly 7 weeks according to my LMP, and I know when I ovulated due to tracking/using OPKs.

The technician told me I was only measuring about 5 weeks. She saw a gestational sac but no yolk or fetal pole. Has this happened to anyone before? I am terrified that this is going to end in miscarriage for me. I’ve read stories of women showing two weeks behind who went on to have healthy babies develop...but there is a chance it could go the other way as well. Any advise would be so appreciated.