questions from a new mom about first trimester

Salatofamily

Hello! I am just writing to get some advice from some of you that have gone through similar circumstances as I have. Last February I lost a baby at about 4 weeks and gave myself about 6 months to recover mentally and physically. On August 11th we found out we are pregnant again and I immediately called the doc to make sure everything was going well in the first couple weeks. He ran hcg and progesterone tests and found out I have low progesterone (7.8) so I was put on the vaginal prometrium pills right away.I had one spotting on one day which the nurse said is most likely implantation. I continued the hcg tests to make sure my levels were rising and they were but by the third test they were concerned because they were "slow rising". My first test was 710, second 1107, third 1413, and fourth 1600. I then was scheduled for an u/s after my last test to make sure everything was looking good. The u/s went great! They found the gest sac, yolk sac, and baby.They said I was actually not 5w5d as they thought but  I was 4w5d according to the measurements of the baby. The nurse went on to say I must ovulate later in my cycle since I have a 31 day cycle and that my levels seem normal now that they have realized I am only 4w5d now. The next day the doctor called to talk about my results and said my hcg levels are not rising as he would like them to but he has seen cases where the numbers do not rise but still produce a healthy pregnancy. The part that made me start worrying is when he said it's all up to the next u/s where I will be 6w1d . He said if the baby is not growing or the heart beat is not found then we are looking at another miscarriage. I haven't had any other spotting or miscarriage symptoms so why does he have to scare me into thinking I might have another one ? Another issue I have is that the nurse gave me hope by saying sometimes we won't see a heartbeat at 6w1d we might find it at 6w5d but then the doc is saying if it's not present by Friday it's a miscarriage!  Has this happened to anyone else? I feel great in this pregnancy except for the normal nausea, tiredness, and super sore boobs.