when you go in for your first sonogram aand they scare you

so 2 years ago I had a miscarriage at 11 weeks. when I went to emergency room they discovered it stopped growing at 5 weeks and I just carried until week 11. needless to say I was devastated. it would have been my first.

at the 1st doctor appointment on August 21 they said I should be about 6 weeks pregnant. on Friday September 1st I went in and the sonographer took this image and said I was only 6. Strong heartbeat! needless to say I cried but I kinda freaked Out because I was expecting 8 weeks not 6 weeks. has this ever happened to any one. she said I shouldn't be worried because I could have just ovulated late...and based on my glow log I should have been ovulating if you count back. but after suffering a few years ago I just need to know if that is normal...