progesterone drop?

My hcg numbers are more than doubling every 48 hours. My progesterone level went from 22 down to 20.2. The nurse said not to be worried because at this point in my pregnancy (5w5d) the progesterone should be in the teens, so it's great that it's over 20, but I am still concerned. Shouldn't it be riding along with the hcg?