Waiting for a miscarriage?

At 5w 5d I went for my first ultrasound and was told the gestational sac was only .48cm and measured 4w 2d. Blood drawn that day with progesterone level a 10.

I know there isn't a mistake in dates because I was using cb fertility monitor and it measured my fertility levels every day for 20 days of my cycle. I also meticulously kept track of days we BD'd and whether it was during a fertile period (checked by ovulation tests and cbfm).

Blood drawn two days later (6wks) with progesterone levels still a 10. Hcg levels at 5,000 - "basically the same" as two days prior.

Slight pelvic cramping on right side (where cramping was around implantation and early pregnancy.

I just want to know what this is. No sugar coating. Dr said could be a slow grower but my progesterone is low and Hcg levels should be increasing. They just stayed the same.

Please share your experiences so I know what to expect!