WHY IS MY CAT ACTING SO MEAN LATELY?

My cat has never really been a cuddly cat, but he has never acted the way he is now. He has been hissing at everything he sees, my dog, my kitten, and even sometimes a houseplant. My dog (a pug) has been in the house for about 11 years before we even got the cat. The cat is about 1.5 years old. And the kitten has been in the house for around a month and a half. At first the two cats didn't get along but then after around a week of hissing and hitting, they both became pretty good friends! The kitten would follow the cat everywhere and they would lick each other clean and whatnot. But all of the sudden the cat wants nothing to do with anyone and is hissing at everything. The only thing I can think of that would make him so mad is that I took a 3 day vacation and then came back. But I had someone feeding him and cleaning the litter box and doing all the nesisary stuff to take care if them and the vacation was like a week ago!