My cat is just different lately.

I've had my first cat for a bit over a year. She's always been pretty hyper and vocal. Enter new kitten about two months ago. Now my first cat seems depressed and tired all the time. Her eyes don't open as wide and she just looks like she's not really there. She's quiet most of the day now. She sleeps for most of the day and has been sleeping in new areas that we don't really see her in as much (like at the kitchen table when we are in the living room).

I'd understand it if she was just trying to get some space from the kitten, but she really seems like she's just not THERE anymore inside her sweet little head. You know when you can just tell that something is different but can't pinpoint it? She even looks different. She looks so tired all the time, like she can barely open her eyes. I'm not really sure what to do. She got her yearly checkup at the vet recently. While we didn't bring this up (assumed she's just tired from having company), the vet didn't notice anything out of the ordinary.

Does anyone have any thoughts or suggestions?

Did you get blood work done as part of your yearly checkup at the vet?  If not, I would suggest you do, and tell your vet about the change in her behavior, because the vet would not know unless you told him or her, and the exam is different if they are looking for illness than if they are just checking to see if there are any outward signs of problems.  You might also check about the background of the new kitten and see if she was ever sick, because even when they recover from an illness and seem perfectly well, there are some illnesses they can still pass on to other cats, things that vaccines do not protect against.  Let us know