Hi again, LaDonna, and thanks very much for your additional and helpful information.
I'm glad you stopped her before she could eat the piece of homemade onion roll, as onions are toxic to cats. It's possible that when you shooed her away from the piece of food on the floor, and out of the kitchen, you startled her, and sometimes when cats are startled, they will hiss and/or growl, just as a natural defense mechanism.
Also, sometimes, after a fright or being startled, the cat will start showing repeated aggressive behavior to everyone in the household, because they don't know how to control this response and still feel anxious, or feel like the same startling event may repeat; this is called redirected or misdirected aggression.
Cats may also appear aggressive or hiss/growl when they are in pain or something is bothering them physically, so it might be a good idea to have her evaluated by the vet, to rule out any medical problems at this time.
Once that's done, and she's given a clean bill of health (hopefully), you can treat this as purely behavioral, and a cat-appeasing pheromone called Feliway/Comfort Zone for cats, might help. Feliway comes in both a spray and plug-in diffuser, and is available at major pet supply stores like Petsmart, and also online. The diffuser resembles a plug-in air freshener, but has no scent to humans. Feliway helps to relax stressed/anxious cats, and is useful in a multitude of situations. For more information, please see: www.catfaeries.com/feliway.html
Even though you didn't have guests during the holiday weekend, a new or different smell, scent, cleanser, noise, etc. may also have put your cat on edge, so give her some time to outgrow this behavior, if you determine it has no medical basis. Continue to treat her as you always have, and try to keep her routine the same. When she hisses or growls, just ignore her and don't approach if it makes her uneasy.
I hope all will be well!