Hcg levels?!

My levels were 540 last Monday. The dr told me that puts me around 4 weeks but according to the first day of my last menstrual cycle I should’ve been 5 weeks. They told me to come back this Monday and test again that if my levels go up then when they get to 6000 I’ll get an ultrasound. This didn’t happen with my first pregnancy which I had 0 problems with so I’m really worried that I’m going to have a missed miscarriage. I’ve had cramps, sore boobs and back pain since before I found out via hpt. I’ve had nausea but it’s come and gone. Is that a bad sign?