Why does my cat pee out of the litter box?

I have a unfixed female 1-year-old cat. She's recently started peeing anywhere but in the litter box. She's peed on my couch, my book bag, and on the carpet. I keep a clean litter box, that she shared with two other kitties. She recently also pooped outside the litter box. I don't know what to do, the vet said there is nothing wrong wih her. I don't want to have to get rid of her, but i'm tired of my things being peed on! Help!