I have a specific question for any OBGYN's. If Conception occur does the progesterone level fail to drop.?

I have read alot of information on hormonal levels and HCG and I am not currently past possible conception to have any HCG in my system. It's a complicated situation and I do not want to take the plan B because it may disrupt my cycle which I cannot have. I am only 20% worried about conception, I have had a hormonal level done today or other reasons..... My progesterone level is dropping. Would this still be happening if I had conceived three days ago....