Is she seeing things?

I tried too look for this on the site before i started a new thread but...i got bored. My kitten is about 6 months(shes not 6 months but shes getting there) My boyfriend has a one year old cat name Kid and they are the best of friends. I take her over to visit all the time, so that she has another cat to play with and shes not bored. Recently i noticed her fur "standing on edge" i cant really explain it but she could be off doing nothing, just walking around and all of a sudden her tail doubles in size. Since i've never owned a cat before i bought a bunch of books on cat care and one book mentioned how cats would do this when they're scared or trying to scare someone off. The thing is, NOTHING is there. Is there a reason for this happening? Im just wondering if somethings wrong with her, but it looks really cute >_<

It is their way of saying they're excited about something.

the question is interesting. If cats sees sometimes something we humans dont see? Perhaps.
But it is probably another story.

Youg half grown cats do often such things. Our cats did so too.
So if she sees something we may wonder, but almost 100% sure she is not sick.

You dont need consult any vet for this.

Maybe there is some scent that she notices but you don't. Sophie will puff up her tail if she smells anything odd in her domain!

Yeah, I've noticed Ollie pretty much has super senses, haha... I always know when he's spotted a bug and it's way before I'd ever notice it... his tail puffs occasionally too and I'd agree that it's probably some smell that we can't detect... isn't a problem and there's nothing wrong with your kitty

My boyfriend's cat reacted that way when he put a poster of a realistic-looking video game character on the wall. Syd hid under the bed for several days. One of the cats I had as a kid would get freaked out by plastic bags, and he would literally jump backwards about a foot or two. It could be your kitty smells something, feels a change in the air, notices something under her paw, or any number of things that we wouldn't notice.