Progesterone is supposed to go up in general BUT it also fluctuates during the day same as any other blood work and depending on the lab which tests the levels. If your progesterone dropped by 10 points then the Dr should run another test as a precaution and if anything put you on supplements. In the past, I had 1 m/c at 9 weeks which was assumed to be because of progesterone issues. Started at 13 and then the Dr (which I dropped) back then had me on Prochieve 8% and tests came back at 18. I was 5 weeks at the time. Still, levels were too low but he didn't follow through with more tests because he saw the heartbeat and HCG levels were up. I ended up miscarrying at 9 weeks. He used the "chicken and the egg" excuse. He was too lax and didn't follow up. If there is any sign of levels not being what they should then precaution should be taken.
This time around, my new obgyn put me on progesterone in oil (PIO) shots simply as a precaution from the time I found out (surprise!) I was pregnant at 4 weeks. My levels at 4 weeks (without the PIO) was 21.5 and then a few days later w/ the PIO went to 44. Now, I am 6w5d and am still on the PIO as a precaution even if I may not need it.
My Obgyn told me that the PIO is offered by most and the best fertility clinics as well because the the oil is best absorbed by the body and at "uterine" level. They have shown best results. Although it is a pain in the rear (literally speaking) I would rather deal with it since it offers support.
Good luck to you and be sure to follow up with regardless. Keep us posted.