It is possible for them just to fall out. I lost a seton that way, and can appreciate your distress. It's bad enough going through the placement, without them falling out soon afterwards.
I'm glad to hear you have an appointment tomorrow, or else I'd say "call your doctor, asap".
However, my feeling is that if you're in that much pain, you need to go down to the hospital now, even though you do have an appointment tomorrow.
Hang in there.
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