My cat used to be gentle and lovely, but now she has turned timid/shy and fierce to my other cats? :( x?

Its quite upsetting she used to curl up next to me but now all she does is hiss and stuff at other cats when they just walk past her and they have done nothing wrong! x shes quite old i think shes the oldest cat out of 3... shes called daisy:{ she lost her sister when she was younger... why is she like this? what shall i do?