8 week ultrasound but couldn't see anything

My last period was aug. 8. I don't have a regular one so my cycle sometimes can be 36 days or so. My 8 week ultrasound was today and they couldn't see anything so I went for bloodwork and the doc called and told me that it does match the ultrasound. Has anyone else had this happen? I had a miscarriage last year so I'm a little freaked out!