Bbt not dropping?

Hi all,

I’m having a very weird chart this month. I started spotting yesterday, and today it became a full blown af bleed. However, my temps have not dropped one bit. Usually my temps will drop 1-2 days before af actually arrives

An additional fact is that I just started on Letrozole, and am actually 2 days late for af before af showed. My ovulation was also confirmed with blood test on cd 22.

Not sure what this mean. Could it be breakthrough bleeding or late implantation bleeding?