As I don't know where from and what flight you are on it would be difficult to be precise.
I can say that if the flt is a 777 and has been switched from a 777G (3 class) to a 777H (4 class) then that would be the reason as the 3 class has 31H/K but the 4 class does not, it has 31 AC DEFG but NO H/K as there is a toilet in that position.