absolutely outrageous situation. you are on the high road, which usually isn't crowded, so have faith in yourself. if it were i, which is different than you, i would approach her and request that we re-visit our conversation. in no way make it personal and push the problem off onto some other situation, such as the plumbing being poor because it is an old building (perhaps that is the situation.) i would explain that i think there are some misunderstandings, that i have appreciated using the staff toilet because of my handicap, that i would be willing to have a plumber come in to correct the problem, or something on that order. remember to deflect it away from her and nothing personal, other than mentioning that you have a gi handicap (i usually keep it simple and say that i have the gi system from hell.)
then again it is the end of the year, but i'm assuming your contract will be renewed. would be better to get it settled now, if for no better reason that you don't fume over it for the summer. good luck!! janet