I’m desperate to know ..

This is kind of a long story so bare with me . My last period was on September 9 and only lasted until the 11 . I was expecting a period according to this app on October 12 but I didn’t get it so I am 9 days late before this missed period I took a pregnancy test because I started feeling constipated with headaches and very tired and got a faint positive . After that I kept testing and all negatives and I’ve always done it with my first pee of the day in the morning . Why is my period late has this happened to anyone before ?