Went in at 6 weeks, measuring 4 - need good vibes!

Gin
So I miscarried in December and am pregnant again, so as you can imagine, I'm a bit nervous, so looking for some good stories. 
I got my period on 1/29/17 and it was a bit longer since it was my first normal one post-miscarriage. I don't even know what my cycle is like anymore due to miscarriage. I started taking ovulation tests on 2/11/17 (day 14) and had some high fertility around 2/12-2/17 (days 15-20), but then stopped testing since I figured we'd just go for it. The next week, I felt "frisky" around 2/21 (day 24), so I took another digital ovulation test, and it was "peak fertility" so we went for it one more time, though I'm not sure if the test would be accurate since I stopped testing for a few days...
I got my BFP for the first time in the morning on 3/3 (would be "day 34", a couple days after missed period), but VERY light. It wasn't really solidly dark until 3/5, and darker still after that.
I went in today, 3/13, (just to ultrasound clinic - doc appt is tomorrow) for 6 week early ultrasound, and they saw a gestational sac and a tiny little yolk sac, but she said I'm measuring more like 4 weeks, which the ultrasound tech said would be more consistent with the 2/21 conception date. But 2 weeks off freaks me out.
I see my doctor tomorrow, and will talk to her more in depth, but I'm just super nervous. It seems to make sense to me that I just ovulated later, which would correspond with the later BFP.
Any insight? I'd appreciate some *positive* stories of course.