Period one day after ovulation??

I use the clearblue digital ovulation kits and right on schedule I tested high on Friday & peak ovulation time on Saturday. Had sex both days but when I woke up on Sunday to find my period I was so confused. Is it even possible to have a period one day after you ovulate? I usually ovulate between day 13-15 of my 26 day cycle (give or take) but have never gotten period w/in a week of ovulation. Didn't think the digital tests could be give a false positive but what else could it possibly be?