I have no clue :/ I hope someone helps you out...I remember reading about if a villager gets sick somewhere...but don't want to give false info Good luck with coco
My guess is she is leaving. Since if you do not talk to a villager(must be outside) for sonetime,they will eventually move. Did you give her Meds? It raises friendship level and probably a way to prevent her from leaving.
I am sure she is still moving. Rosie was sick for me one day before Festivale. The next day she was completely fine and skipping around in her Festivale clothes. She never even acknowledged that I gave her medicine or mentioned that she was better, how mean of her.
So I guess if there is an event (and I guess moving would fall under that), they will 'ignore' the sickness.