Progesterone dropping

I’m 4 weeks, 3 days. I had my progesterone checked last week and then again this week (four days later). It dropped from 22.3 to 17.9 in four days . Is this something to be concerned about? My doctor says I’m still in the normal range but said she could perscribe me progesterone to be safe if I wanted. I am going back in in two days to get my levels checked again. I’m considering waiting until then to decide, but I don’t want to risk it dropping too low over another three days. Has anyone had a similar experience?