Help new 2 the app!!

Hazel😍 • Mother Of 2 Princesses 👑👑 4 Angel Babies 👶🏽👶🏽👶🏽👶🏽

Hi! I'm new and in trying to figure out why this app keeps saying my next cycle is due on the 21st of this month if my last cycle started on the 16th of December and was due yesterday? I put I had a 28 cycle day when I set the app up so I'm totally confused.