I think my kitten is sick?

She's 4 months old, and the entire time I've had her shes never acted like she is today. She's always hyper and running around and getting into mischief. But today she's been hiding all day. Right now she's curled up hiding behind the tv. She never goes bk there. She's basically been out of sight all day, sleeping. Usually, when I pick her up, she fights it with all her might (the little rascal hates to be held) but just now I picked her up and she just kinda layer there limp in my arms. Just like, she wasn't fighting it or anything, which she's never done. I think she's sick or something. What should I do, if there's anything I can do?