It is getting way out of hand..

When I first brought Saedie home, my other cat Millie accepted her fine. She didn't love having a new kitten in the house but she tolerated it. All of a sudden Millie hates Saedie. She growls if Saedie is across the room from her. Millie really hates me now, too. If I try and pet her she growls and hisses at me. She won't even eat if Saedie is anywhere near her. She hides in the closet all day so I bring her food and water so that I know she is getting enough food and water.

Millie started acting like this out of nowhere. What can I do so Millie can at least tolerate Saedie again?

She is about 4 months.

i agree with the heat thing, when forest went into heat at 5 months old some of the other cats went crazy. if not maybe get millie a vet check just to make sure all is well.