HCG levels rising but not doubling, Progesterone level dropped .2 points? Someone please help!?

My LMP was on Jan. 29, I was due for AF on March 1, I never got it but after I had been late I started getting mild cramping, breast tenderness, and I was very tired. I thought it was my period coming on but it never came. I tested my first time on March 8 -FMU, but it came out negative. I still had the cramping and sore breasts and still tired so I decided to test again on March 14 after work. I got a BFP but it did take a min or so to come out clear it was positive, I tested again the same day and it took about the same amount of time. I tested the next day and it came back faster but not right away, I went to the Dr, to get the pregnancy confirmed through a urine test and it also came back positive. A week later I was worried because my breasts weren't as sore as they had been so I tested again and it came up positive right away. By my last menstrual period I should be 9w4d but I have always had irregular periods and I do not know when I got pregnant because I do not track my ovulation because we weren't trying for a baby. I had to get some blood work done on Friday April 1, and my HCG levels were 10,393 and my progesterone was 8.7, I went for an ultrasound today April 5, and they only saw a gestational sack and it was empty. The sack only measured to be 5w3d, I was very upset but thought maybe I was only that far along since I got the negative test the 1st time I tested on March 8, but to see nothing in the sack was a concern until I read about many women also not being able to see anything that soon. My doctor said it could be a very early pregnancy or a Blighted Ovum, He wanted sent me to get more blood taken to make sure my levels of HCG was doubling, I got that taken the same day. My HCG was 17,527 and my progesterone was 8.5, my HCG didn't exactly double but it is 3, 259 shy of doing so, I have also read that it takes 96 hours for HCG to double when it is over 10,000. That is 4 days and that is exactly how long it was between my blood work. My Dr. still doesn't know if it could be a miscarriage that is going to eventually happen on a really early pregnancy, The HCG not doubling kind of worries him but he still doesn't know fully. My Dr wants me to repeat the blood work on Monday and if it looks good he is going to schedule another ultrasound. I am hoping for the best but am preparing for the worst. What does anyone else think, I don't want to loose hope and I also do not want to worry/stress since that is the worst thing to do right now even though it is really hard not to think something is wrong with your baby when your playing the waiting game.