It could just be normal. Mine will sometimes go many days on a hunger strike before they start eating again.
When I have a shoal, I still try to introduce food daily tho so they don't prey on each other.
My Serra's, which I keep alone in tanks, sometimes go a week without wanting to eat. When they fast I don't bother trying to give them food every day but maybe every other day or two.
What's the temp of your water? If it's between 79-84 and you have good water params I wouldn't worry about it yet. Piranhas can go a couple months without eating but make sure you get any water quality and oxygen level (in the warmer temp of the year, make sure you have your filters breaking the water etc) questions out of the way and I think they'll eat again in a couple days.
