It's the little demon of loneliness. At first she tried socializing because the demon was small. Then when she was in the bus she felt lonely so the demon began to grow. In the third picture she is walking within people, but this time she isn't even trying to socialize and the demon has gotten bigger. And in the last picture the demon is now almost human sized which means that it will be hard for her to get out of the problem of being depressed because of her loneliness. She hasn't even slept because she might be crying every night for always being alone and that demon has only been getting bigger and bigger.