TMI - But Need Advise

I've had my period from the 2nd January to 6th January . Normal 5 day period .

Today is the 7th , everything was fine . Had sex with my husband. All fine , peeded after sex and nothing weird .

Few hours later , I went to toilet and noticed that when I wiped . There was a dot of blood . Never has this ever happened . So I decided I'll put a pad on . Rechecked, I think I'm spotting . Why is this happening . In the last god knows 15 years . This has never happened. Does any one now what it is ? Or why this happened ?