Update: had a plumber out and he unclogged the sewer main and said it was almost certainly the excessive amount of sanitary items flushed down there. I believe that these sort of things don't clog the toilet itself, but can get all stuck together in the outgoing sewer pipe, which is exactly what happened.
I messaged the guest to ask (politely as possible!) if she had flushed sanitary items down the toilet, and she apologied and said she had, and didn't know it would be a problem.
Luckily the plumber only charged $135. Can I ask the guest to cover it?
This whole situation makes me so uncomfortable, but I do want to advocate for myself. Also, how do I review her, as she has been fine other than this issue?
Also, I have now posted a sign above my toilet and will add it to the house rules as well. Sheeesh!