You probably have separate Grey tanks for bath/shower and kitchen sink. I have a half bath in my floorplan and two separate tanks.
Between those two activities (shower and doing dishes) one tank would fill pretty quick.
Either when you are in the shower or when she is doing the dishes, one of the drains will stop flowing.
Go outside to the valve handles ( get dressed first if you're in the shower
) and open one valve and see if the full sink or shower drains. Then label the valve so you know which one is which in the future.
Good luck
RMc