Who had a period on the 1st of January and ended on the 7th

Michaela
Ok according to glow my fertile window is the 15 th to the 21st I am not sure what day I ovulated on because every time one looks Dar then the next one is light the dark again. My period started on the 1st and ended on the 7th. When should I be starting my next period if I go on a 28 to 32 cycle?