My Cycle Is On The 18th Of Every Month I

My cycle is on the 18th of every month I had some cramping on the 1st of Nov, spotted on the 18th of Nov and found out on the 23rd after taking a test. I set and app. And at the beginning of Dec the nurse said I was 6wks and some days. My first app was on the 19th and I should have been 8wks and some days. They did my first ultrasound and said the baby was so small they couldn't see a heartbeat so I might be earlier than we thought more like 6wks 4 days then the AN said my baby might have just stopped growing is this something I should worry about???