I just got a new kitten and now my original cat hates me?

I got a new kitten a few days ago to play with my year and a half year old snuggle-bug cat, but now my original cat won't come anymore when I call her name, and sometimes she hisses at me for no reason. I know there is an adjustment period, but I really had the best of intentions that my original cat have a playmate. How long will it take to adjust????