Does anyone start their fertile window almost immediately after their period??

Holly • Cat enthusiast and attorney who specializes in criminal defense, family law, and general civil litigation.
I always thought the fertile window didn't start for about a week after your cycle. 
My cycle technically ended on 12/31, and Glow is already predicting my fertile window to start tomorrow, 1/4. 
I took an ovulation test today, and it looks pretty positive to me. 
What gives??