Question-about charting bbt

So i came off bc 6 months ago and my periods have been irregular- I've only gotten it twice since then and sorry tmi but some brown muscu stuff on March 8th. I started charting my temps 4 days ago and my temps have been going down everyday. The first day it was 97.18 then 96.99, 96.98 and 96.87 today. What does this mean? I thought it was suppose to go up and down like a chart? I read online that it means my progesterone is low, which is something I've always suspected. Can someone kinda give me some insight, what do you think it means and is there anything I can do to help it? Thanks!