What's wrong with my cat /an/? ;_;

Pic related, it's my cat. She's usually pretty energetic and friendly (well, only friendly to me, but she's always been like that), but lately she's gotten very thin, hardly eats, sleeps all the time, is very antisocial and hardly ever purrs anymore. It's almost like she's depressed or something.

I'm taking her to the vet today, but I thought I'd ask you guys too. Is it an infection or something? Is my cat just depressed?