Help me someone! According to lmp I should be 7 weeks!

Hey everyone my last menstrual cycle was December 13,2017 12/13/17. I got a smiley face opk on the 29th and 30 and baby danced on 29,30,and 31. My cycles are average 31 days. I went to get my beta hcg levels done 1/17/18 they were 548. Then before then I had a ultrasound thinking I was 5 weeks 3 days at a private ultrasound place. The ultrasound showed nothing!!!

I went today for my very first prenatal apt and according to there quick sneak peak ultrasound I’m 5 weeks 3 days! Seen the baby and the heart beating on the screen. I didn’t get a chance to hear anything yet bc she said next apt will be more thoroughly....

So why according to my lmp I’m 7 weeks and according to the ultrasounds and my hcg a few weeks ago I’m a lot earlier than expected!

Should I worry?