I had a miscarriage in between my first and my second child and I had just got off of the bc and got pregnant right away. I was about 4 weeks along and I bled really bad and then I went to the bathroom one day and I felt like I passed a blood clot but come to find out it was the baby. I wasn't emotionally attached yet so that was good for me. I went to the doctor shortly after that and they drew blood weekly to make sure my HCG levels went back down to zero. She specifically told me not to have sex until I got two regular periods but of course my husband thought otherwise. So about the 4th week of the visit my HCG levels went back up and the doctor was upset because she thought i had had sex. The 5th week I went back and my levels were at about 1200 so she said I was pregnant.
They monitored me all throughout my pregnancy, I had ultrasounds quite often during the first trimester and I progressed regularly. It was a really confusing time because I wasn't sure that I was pregnant until we started looking at ultrasounds and checking for heart beats. The time frame they had for my delivery was very accurate because they had monitored me from early on.
What I would suggest is for you to get the vaginal ultrasound, that should determine if you are pregnant and at what stage. It can be nerve wrecking because you don't know what to think right now but the doctors should be a lot more proactive about checking to see if you are pregnant or not and giving ultrasounds to make sure that there aren't any remnants from the previous pregnancy. Also, what the doctor did was put me on Progesteron (i think that's how you spell it) to make sure I wouldn't miscarry again. You should check and see if that's an option for you as well. I wish you well and hope all is well with you.