Should I worry being a week behind?

I had my ultrasound on August 29th. She said I was measuring 6w2d and had a heartbeat of 118. BUTTTT looking on the ultrasound it said 7w0d. Going off my LMP I would be 10w2d today but going off the ultrasound I'm 9w3d today so I'm a week off. I'm assuming I ovulated a week late? Which would make sense considering my cycles were 30+ days prior to getting pregnant. I don't think the month I got pregnant would be the month my cycles got regular all of a sudden.

But one thing I was kind of worried about was the heart beating so low. All the other post I read at that time in their pregnancy had a higher heartbeat. Should I worry?