Just ask her why she would think it's you. If she isn't familiar with the intricacies of your output and is just making a guess as to who the toilet clogger is, maybe she needs to be educated. But nor have you posted what the dynamics of your relationship are or whether it really matters if you don't use a bathroom you never or rarely venture into.
Quite frankly the answer you gave likely sounded like an admission to her in all likelihood.
Situations like these have come up in various office buildings I have worked at and the protocol for handling it is always the same, to post a sign over the toilet as to what not to put in it, rather than bar someone from using it. But if you are staff you get to use staff toilets- if you aren't you don't get to use them. Or so I would think.If on the other hand you are using some handicap privilege to access toilets you otherwise wouldn't have access to, others may have complained. This is all just speculation on my part because I don't know what all the facts are. I know that I have asked for and received accommodations as far as bathroom privileges when I had UC from various schools I attended. It's less of an issue since I got a J Pouch. Anyway good luck with it. I would have handled the situation differently but I don't walk in your shoes-plus I don't generally react very well in such situations when they arise. Such as the other night when my condominium neighbor parked her car in my guest parking spot. I am a speak his mind kind of guy for better or worse, but I think it can be worse in some situations.