hCG Levels vs Ultrasound - how far along?

Hi guys! 

I am new to this board (my 1st miracle) so I am a little concerned/confused and a little bit of a worry wart! So I was wondering if I could get some insight from y'all to hopefully ease my conscience. 

So I took the pregnancy test on 8/26 and got a BFP (I took 3 just to make sure!), and I had my first doctors appointment 8/29. They ended up drawing blood to confirm the pregnancy and check my hormone levels. My hCG levels were 1353, which according to the chart the lab provided put me between 5 & 6 weeks. 

They scheduled me for an ultrasound on 9/4 to give me the due date and age, when they did the ultrasound they said I was 5w0d and there were no signs of a miscarriage, so I may just be earlier than they first thought. I got more bloodwork done, and I just received the results, my hCG level is 7269 which would put me between 6 & 7 weeks. 

Since my hCG levels are a week or two ahead of the baby, should I be concerned? I am worried the baby isn't developing correctly, but I am not sure the worry is warranted. 

Has this happened to anyone else? If so, did you have a healthy pregnancy? 

Also, I am not sure if this matters but I have very irregular periods which come around every 2 - 3 months, and this is my first pregnancy. 

Thanks in advance!