My bf predicted my period???

Lidia

My boyfriend predicted I was going to get my period. It was soooooooo crazy. We had talked on Sunday and he asked me if I was on my period yet and I said no. Then, I asked why he thought I was, and he said because I usually get my period on the 4th or 5th and that isn't true. Usually I get it on the 16th or 17th. The next day, I get my period🤔 someone explain how???