Any similar situations?

I had my first prenatal appt yesterday Sept 10th. I should have been 8w1d according to my LMP which was Jul 15th. When the doctor did the ultrasound, she said I was much earlier and dated me at 6w1d. She said this was normal especially because I have irregular periods. What I am worried about is that the baby isn't growing because I still got a positive pregnancy test 2 days before my missed period on Aug 13th. So that would put me at 2 w 1d when I got a positive test and that doesn't seem possible if I was really two weeks behind what I thought I was. Anyways, I went back today because I had quite a bit of clear watery discharge that later turned slightly brown and constant cramps all day. The doctor did another ultrasound and could still see the heart beat( can't hear it yet because its too early) and said everything looked just as good as it did at my appt yesterday. I've been especially nervous with this pregnancy for some reason and I know if a miscarriage is going to happen, there isn't anything that can be done but I just wondered if any of you ladies have had this sort of thing happen before. Thanks for taking time to read all this!! Hope everyone else's pregnancies are going well ☺