Cats suddenly hissing and growling at each other??

I have 2 cats. A 1 year old cat named Lucipurr and an 8 month old cat named Alice. Alice showed up to my house back in November. It got cold, so we took her to the vet, got her shots, and let her indoors. For months, Alice and Lucipurr played together and generally got along very well. Alice just recently gave birth to 4 precious kittens 2 weeks ago. But towards the end of her pregnancy, Luci started hissing at her. Alice and her kittens stay in my bedroom. Sometimes Luci sneaks in my room and starts hissing at the kittens and growling at Alice. Why are they suddenly not getting along??

(P.S I also foster kittens and both Alice and Luci get along with the kittens just fine. Help!) 

I'm only guessing but there is a perhaps that Alice no longer smells the same to Luci. Non- recognition aggression happens to some cats when one the other returns from a trip to the vet and smells different. One of the primary senses for a cat is it's sense of smell, if a cat doesn't smell like itself it's a stranger. It is fixed by separating (or bathing) the cat newly returned cat to allow them to become friends again. In this case it may be the change of hormones making Alice smell like a strange cat. 

I'm assuming that in the case of the kittens, it may be territorial aggression. Cats are territorial creatures, even kittens are strangers. Cats need to be introduced to each other slowly. 

Also, cats which are fixed tend to be less aggressive and have less behavioural issues than those that aren't.