High fertility the day I ended my period?

I started my period on the 15th and it ended on the 20th. I tested the day I finished because I felt cramping on the side of my ovary and thought maybe I was ovulating early but I’m not ovulating yet.... I got a flashing smiley on my Clearblue test. has anyone ever had high fertility the day they ended their period? I’ve been looking online trying to see if that’s nirnal because I thought I wouldn’t have high fertility til around day 10 of my cycle which is 4-6 days before ovulation?