ttc after chemical progesterone ?

Had a Chemical which ended. Only had bleeding for 4 days. I am now actively TTC dr prescribed Prometrium 100 mg a day and said to start after i get a + test, but also can start 3dpo. I am not sure what to do my chemical I had very slow rising hcg numbers and then low progesterone. This was my first cycle off IUD. Any help or advise ?