Help understanding bbt chart.

Ly
This is my first month using the chart feature and I'm still trying to understand everything that goes into ttc. I've noticed that my chart had a fairly significant dip then rise glow told me this is a sign of ovulation but I ovulated on the 15th this was confirmed by an opk, thee dip I'm referring to is on the 20th. I haven't been doing anything different my house is the same temp always. I would love some insight as to what this could mean?