Low Progesterone?!?

So apparently I have low progesterone. At 5 weeks my progesterone was at 12 and liked it to be at 15 in the fifth week. Got put on progesterone supplement to take once a day, which I've done since. Got another blood test 4 days later and they said my progesterone level went down to 8! Ahh! I'm so nervous. They said the blood the baby is growing like it's supposed to be. But, now they have me taking the supplement twice a day. We have an ultrasound in four days to hear the heartbeat. I am just praying everything turns out okay!!! Any advise??