Diarrhea and vomiting with a change in appetite can be due to stress, a sudden change in diet, eating things she should not (especially if she goes out unsupervised or is fed lots of table scraps), parasites, viral or bacterial infections, inflammatory bowel disease or even a food allergy. But it can also be related to metabolic organ disease (such kidney or liver disease) due to organ wastes irritating the gut. Rarely a mass in the gastrointestinal tract can be the cause.
Because your other cat seems fine an infectious cause seems less likely.
I am concerned that she seems lethargic too, she bears close watching.
Since this has lingered for more than 24-48 hours and she isn't herself having her seen today would be best.
If that isn't possible I have some suggestions to try.
I would not offer any food for now. This should help stop gut spasms and restore normal gut motility.
Small amounts of water given frequently are fine as she needs fluids after all that she has lost with diarrhea. You can give her clear broth or pedialyte to replace electrolytes too.
Typing more now...