I'll do my best to answer your question. If you're available for chat, let's get started.
Hi! I'm sorry to hear about this and I'm sure you're very worried.
If the rubber band is just all rubber (no metal) and the baby is fine now (no choking or difficulty breathing or swallowing) there's really nothing to worry about. It should pass without difficulty. You can watch for it in the diapers but it might be difficult to find.
If there was a metal connector on the rubber band it might be prudent to get an xray of her belly in a couple of days to ensure that it has passed out of her. The metal will show up on xray and this would ensure that it didn't get stuck somewhere and that the metal doesn't corrode over time.
Of course if she is having difficulty breathing or can't swallow her saliva or drinks or food she needs immediate medical attention.
Anything more I can help you with?
If this information has been helpful to you and I have left nothing unanswered kindly leave positive feedback prior to leaving the site. If you need more information kindly reply.