Excited and Confused!

So, I'm excited and Confused I'M OFFICIALLY PREGNANT..... I got pregnant last Sept 2015 and lost at 6ish weeks. So my period was regular every MONTH BEFORE THEN around each month on 20th-23 and would last 5days, not heavy either. Then I believe after my lost of my first baby, my first period was in November. I feel so stupid cause I can't recall when and i normally chart it. My boyfriend and I had intercourse around new years or first week of January 2016. But I just went to my first ultrasound sound February 16 2016 and it was at a free funding clinic seemed the machine was older. I was really dehydrated so it was hard for nurse to see or measure. (but I heard heart beat around 110 of baby, and was normal and healthy) From what the nurse could measure from she guessed I was 6 weeks or so. The sack measured I believe 1.1cm. I have had food cravings a lot longer then what she says I had intercourse which would put it January 15ish.... But my boyfriend of 6 years and I didn't then it's impossible because he truly was out of town and has been since today. CAN SOME ONE PLEASE GIVE ME PIECE OF MIND AND HELP ME. could the nurse be off on the weeks, could it be because I'm a pretty thin person, I'm 5'9 only 125 pounds, and my periods were trying to get normal again and not having a period in December.... I did smoke sigs up till I found out and stopped. Could my fetus be growing slower? What do I do?