HELP! What's wrong with my cat?

Around 6 weeks ago, my boyfriend and I adopted Boo (1.5 year old kitty) from the local animal shelter. We did our routine vet checkup, she was fine (other than worms). Almost three weeks ago, we adopted another kitty, Cortana. Both Boo and Cortana are female. Cortana is 7 months old.

When Cortana came home (not from the shelter, from an old friend who is not a cat person), Boo immediately took a liking to her. Cortana hid for about a day to two days. Cortana hissed (never scratched or attacked) at Boo. Within three days, the two were playing with each other.

I looked up what people considered "playing"--They pounced on each other, no "posturing" (except one time). No hissing or biting or scratching when playing, no "hot-oven paw movement" on each other. It was legitimate "tag" almost. They would take turns chasing each other. They'd sit on the bed with each other, both purring.

History on Boo: When we first got Boo, she was a lap kitty. She'd coo at the door when we came home, constantly wanting in our laps, fluffing up our laps, etc. She has dry skin according to the vet (she came home with dry skin, has continued to have it). Even before Cortana came into the picture, when Boo would shed she sheds in clumps (little black patches of fur here and there on the floor).

Ever since about a week ago, two weeks after getting Cortana, Boo's mood has all of a sudden changed. She has a teensie bit of eye discharge (no longer present in her eyes today). Her fur doesn't look...well, well-groomed, I suppose. Right below her neck it's like it got some kind of water on it, so it's matted? (not sure how to describe that). Cortana didn't attack her or bite her there--I groomed Boo last night, and woke up this morning and it's there again. It just seems like Boo's coat isn't quite as shiny as before, and in looking at her eyes...it's just she's got that faraway look to her. She no longer is enthusiastic to play with Cortana (who tries her best to nuzzle up to Boo, and Boo is no longer having it, she just walks away or ignores it). Boo doesn't hiss, bite, scratch, or flatten her ears at Cortana. I can't tell if Boo is eating (Cortana is a pig). We've tried putting Boo in our room at night (without Cortana) to see if she'll eat anything--the food has remained untouched. She didn't get off the bed to my knowledge, either--she hasn't gotten off the bed any of the nights she sleeps with us for the past two nights. She won't play with the red laser, won't play with hair ties, nothing.

I'm just so lost. Boo used to be a loving cat. She was loving even with the introduction to Cortana. And then last Tuesday, just a polar opposite change. Half the time she walks away when we try to pet her (this got a bit better yesterday, she walked up to me and rubbed against my leg)...but her playfulness is nonexistent.

I can't tell if she's sick, depressed...unhappy with the new kitty. It's snowing outside so I can't make it to the vet today but it's supposed to be warmer later today (hopefully it melts). I'd welcome any advice or help.

Thanks.