I was told my sac was measuring small at my 7 week ultrasound and they scheduled me 2 weeks later to recheck it. It was still measuring small but there was growth and the baby was growing as well. The doctor seemed unconcerned and like you, I checked online and started to freak out.
I'm now 25 weeks pregnant and all is going perfectly so try not to worry to much ;)
It has been my experience that the doctor will tell you that there is something to worry about when there is not. Instead your doctor told you not to worry. That in itself is a good thing. Things in ultrasounds can be off 3 days either way so if the baby measurement was off +3 days and the sack was off -3 days you are with in 1 day of each other. At 14 weeks I could lay on my back in complete silence and feel tiny tiny flutters occasionally. You will miss it if you arent paying full attention to it. Very soon you will destinctly feel your baby move and will feel a lot better. Please do me a favor, when you are farther a long in your pregnancy, dont forget to come and give other people some incouragement about this subject. There will be people in the same situation looking for moms with happy endings. Good Luck.
The HB was at 150 beat per minute and has been consistantly strong.
I had a similar thing occur in February of this year. I was about 7 or 8 weeks but the fetal pole measured a week behind and the sac two weeks behind. A heart beat of 120 was found at the time. The Doctor told me to come back in two weeks for another ultrasound but a heart beat was no longer detected. The sac only grew one week as did the fetal pole in that two week time span. You are much farther along than I was so I would try to be optomistic. You have made it beyond the 1st trimester so I'm sure that is a good sign.
God Bless