Kitten suddenly out of sorts

I have a 7 month old kitten who isn't herself tonight. She's been perfectly normal all day, playing with her sister and napping. Tonight, I noticed shewas slightly unsteady on her feet or when she was sitting down, and she seems lethargic. That being said, she did accept a cat treat I offered her, and she ate a few bites of her food, but she was pretty wobbly the whole time unless she was laying down. Right now she's sleeping on the base of her cat tree.

As for her health history, she's a 100% indoor cat. She was spayed at 5 and a half months. She had a bad allergic reaction to her second set of vaccines, but she's not had any problems aside from that. Could her current behavior be something to worry about, or am I just being paranoid over a kitten that's just sleepy?

I'm not one to run to the vet at the drop of a hat, but this is definitely concerning me.  I would keep an eye on her for the next little bit and if there's no improvement, I'd get her to the ER vet immediately.  She could have an inner ear infection that's affecting her balance or it could be something more severe.  At the least, call your vet's office and listen for after hours instructions.

I'm staying up to keep an eye on her. I started going through the motions of getting to bed to see if she'd go with her regular routine, and she did. However, she still seems almost dizzy and acts like one of her ears is bothering her. I grabbed a flashlight and took a look at both ears, but they look clean and healthy with no redness. 

My suggestion is to take her to the vet. It is always better to be safe than sorry.

Just a quick follow up: I took her to the emergency vet, but they weren't able to find anything wrong with her. Today she's acting like her old self and playing vigorously. I'll continue to keep my eye on her, but as of right now she's doing very well. Thanks to everyone for the advice.