Behavior I think my guinea pig is depressed

My guinea pig named Kenley worries me. Off and on since I have had her (almost a year), I have wondered if she is depressed. She is a year and a half old and lives with 3 other sisters in a jumbo C&C cage. She is the second oldest but the second smallest. Her two younger sisters pick on her. Tilly, which I have written about, bullies Kenley a lot. She has calmed down recently, but now the youngest sister, Lola picks on her. My mother said she read that guinea pigs pick on depressed guinea pigs. So now we are wondering if she is depressed, or if being picked on makes her depressed. She loves to be out more than be in her cage, and we worry its because of being picked on so much when she is in there. She constantly sleeps when she is out, which we know is good that she is so comfy with us. She eats fine, drinks fine, still loves treats. I just want to make sure she is happy.