Due date is off!

So I went in for my 3rd!! Ultrasound and I'm 5 weeks and 6 days. By dates I should be 8 weeks... So if that's true I got a positive on a hpt at a little over 1 week pregnant?!? Is that even possible? She couldn't see a clear heart beat but she thought she saw some flickers it's just a little to small to tell. And they took more blood so I will know more Monday. I also have a large cyst. So I have to take it easy.