Cat hissing at kitten after playing together all day?

we recently got a new 3 month old kitten and we have a 8 year old caf. they slowly got used to eachother and could be in the same room without a problem. today they were playing and sniffing eachother, all of a sudden they meet in a room again and he hisses multiple times scaring her. after seeing if he would... show more we recently got a new 3 month old kitten and we have a 8 year old caf. they slowly got used to eachother and could be in the same room without a problem. today they were playing and sniffing eachother, all of a sudden they meet in a room again and he hisses multiple times scaring her. after seeing if he would settle down and keeping them seperated I removed him from the room as he was still hissing. I dont understand what happend, what should I do?