AF or IB?

My period usually lasts 4 days. it usually starts off heavy and gradually gets lighter. this time I only got my period for 2 days and it was super light both days. I got it on the exact date glow estimated I would which was about 15 dpo so I was sure it was AF. I also usually get really bad cramps on the first day of my period but I didn't get any this time? anybody know what's going on?