It's also not uncommon to have a "24 hour flu", which aren't actually caused by flu viruses, but result in flu-like symptoms for roughly a day, then a rapid rec

It's also not uncommon to have a "24 hour flu", which aren't actually caused by flu viruses, but result in flu-like symptoms for roughly a day, then a rapid recovery. Sounds like that was the case for OP, and I doubt elderberry had anything to do with it.