Why does my cat pee in the shower?

I have two cats, one neutered male and one spayed female. They are both 1½ years old and have been living together most of their lives (not siblings). We keep the little box in the bathroom and have never had problems from either cat, until recently. The female cat (i am pretty sure its her and not the male, but not 100%) has occasionally been pissing in the tub of the shower. I know sometimes cats will piss on things when they are mad at you but nothing has drastically changed in their little cat worlds. Could there be an explanation on why she is pissing there? How can I get her not to pee there?

thanks in advance